Darwin's innovation over previous evolutionary theorists was to provide a workable (and still largely correct) mechanism through which biological change could occur. However, in Origin he does not dive straight in, and the term "Natural Selection" is not explained until the fourth chapter. At every point Origin is meticulous in the forwarding of its argument - each chapter contains sub-headings of the subjects up for discussion and a summary of the points raised at the end. He was clearly anxious to leave no room to be misunderstood, misinterpreted or misrepresented. That he has been all of the three has been consistently the fault of his opponents rather than the text itself - the first two editions at least.
Darwin saw that the crucial raw material for natural selection - and this is as correct as it was in 1859, as it was billions of years ago - is variation: "These individual differences are highly important for us, as they afford materials for natural selection to accumulate." Darwin was attempting to produce a work of 'popular', or at least accessible science, and rather than confront his readers with the alien world organisms in the state of nature, he begins by introducing the concept of variation through that which is found in domesticated plants and animals. The mechanism through which variation is used to accentuate change in domestic animals and produce varieties - i.e. Artificial Selection - also provides a useful parallel to the workings of Natural Selection.
Mankind has been domesticating, breeding and changing the nature of organisms under his control for thousands of years - sometimes accidentally, but often deliberately. One of the key elements of the switch from nomadic hunter-gatherer societies to settled agricultural communities (the first instances, we now know, being around twelve thousand years ago) was the domestication of both plants and animals. Domestication is not simply the socialisation of individuals, but the selection of traits over generations desirable for the human owners. Darwin recognises that developmental and environmental conditions could only go so far to explaining variation under domestication. Of domestic plants he notes that by examining the development of pollen and ovule one can show that "variation is not necessarily connected, as some authors have supposed, with the act of generation." Clearly there are hereditary factors at work, though at times Darwin does slip back into Lamarckian explanations for variation, attributing use and misuse for changes over generations. For example, "The great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with the state of these organs in other countries, is another instance of the effect of use."
Largely though, he stresses the importance of inheritance of character in both animal and human pedigrees: "Perhaps the correct way of viewing the whole subject," he concludes, "would be to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly." However, "The laws governing inheritance are quite unknown; no one can say why a peculiarity in different individuals of the same species, or in individuals of different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not exclusively to the like sex." It is notable that these examples (and others) that Darwin gives of puzzles of inheritance are among those solvable with the simple application of Mendelian Genetics (see Missing Links and Further Reading). Though Darwin did not have the answers with regard to inheritance, he was certainly asking the right questions.
So Darwin establishes that characteristics are often inherited - a far from radical claim - and he goes on to argue that different animal breeds have descended from an original wild stock, not, as some believed at the time, with each descending from an individual wild prototype. In, for example dogs, this is confirmed that however different the breeds they all seem capable of interbreeding. Indeed, writes Darwin, the naturalist tends to know "far less of the laws of inheritance than does the breeder." And from this understanding follows the breeder's selection: "The key is man's power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him... Breeders habitually speak of an animal's organisation as something quite plastic." However, as always the raw material of variation must be there first, as man "can never act by selection, excepting on variations which are first given to him in some slight degree by nature." Artificial Selection, like Natural Selection does not induce variation, it can only work with it. Darwin notes that in artificial selection it often becomes a process of exaggeration, due to the values humans have often attached to novelty and the common use of "monstrosities" from which to breed pedigrees.
It is a short step from variation under domestication to the second chapter of variation under nature. Though less obvious to man, any selection pressures aside, there is no reason for variation in the state of nature to be any less frequent than that in domesticated organisms. In reference to particular genera of organisms, Darwin uses the word "polymorphic" ('many forms') used for species that "present an inordinate amount of variation". The term "polymorphism" has since become a central concept in the study of genetic variation and work in the last few decades has shown the huge amount of polymorphism across a wide range of species - in genes that are expressed as physical features and behaviour, as well as in regulatory and 'junk' sequences of DNA that do not 'code' for a particular traits. Incidentally, this idea of polymorphism of genes also provides a solution for a problem faced by Darwin - that of how to define a species as opposed to a variety. Whilst a species can still be approximated in the way that Darwin perceived it, (that of a group of organisms sharing common characteristics and being capable of interbreeding), it is also now perceived as a common 'gene pool' that members of the same species can potentially share. But for Darwin the 'problem' of defining a species held a more pressing point, one that was not even necessarily a problem at all, but rather grist to the mill of his argument: if there is a dispute as to where individuals belong in terms of species and variety this points strongly to the existence of 'intermediates'. And the existence of intermediates points to potential speciation. This is not to say that suddenly there emerges an individual who is of a new species, but that populations of individuals can become segregated from the others in the species and over the course of time evolve into varieties and eventually new species.
But how, and with what driving force, would this process take place? Darwin rejected a Great Chain of Being or even a Lamarckian 'escalator' on which there existed an innate drive in organisms towards complexity (a view also favoured by Erasmus Darwin). Nor, unlike Artificial Selection, was there an active force choosing which breeds and individuals to favour. Key to Darwin's solution is the "Struggle for Existence," a struggle in which the figure of Thomas Malthus looms large. Malthus (1766 - 1834), economist, sometime country curate, and for many the father of modern demography, had published An Essay on the Principle of Population in 1798, and Darwin had read it forty years after that. Malthus argued that populations tend to increase faster than the supply of resources available for their needs. The resources that are available, therefore, are unable to support that level of population growth and mortality results keeping population size checked. He postulated that if a flock of sheep were allowed to breed unchecked from the number that could be support by one acre of land, they would cover the area of the globe in twenty- six years. Despite his animal examples, Malthus' concerns were socio-economic, and he formed his arguments in part to bolster the case against the use of what he saw as excessive social welfare. Malthus wrote that "Nature is so fecund that any careless attempt to alleviate poverty will encourage insupportable increases in population, and would thus only exacerbate the suffering it is designed to relieve. As far as I'm concerned, nature is unimprovable. Social reformers should therefore allow events to take their inevitable course and let war, disease and starvation reap the surplus." To a lesser extent Malthus also saw the possibility of the limiting effect on population of marriage and "moral restraint."
Darwin was set against this laissez-faire approach to social welfare, but he could not escape the logic of Malthus' maths and its implications for organisms in the state of nature: "It is the doctrine of Malthus applied to manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them." The apparent superabundance of food in nature was in fact illusory, and therefore, "more individuals are produced than can possibly survive," so "there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life." Indeed Darwin had seen two of his children die in infancy and another at aged ten - they had failed in the struggle for existence against the physical conditions of life.
Many perceived a brutality in the world painted by Darwin, a nature 'red in tooth and claw', but Darwin himself often seems at pains in Origin to soften the picture: "I should premise that I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny." Darwin was not merely being sentimental, he had seized upon the crucial truth that natural selection works by differential rates of reproduction - it is no good surviving unless you can produce offspring, and importantly fertile and healthy offspring. A good indicator of success would in fact be not number of children but number of grandchildren. Darwin also understood the "mutual relations of all organic beings" - a web of life not merely a vast angry amphitheatre of competition. But perhaps there remains a hint of sentimentality, or perhaps concern that readers would feel alarmed by the rigours of nature. Concluding the chapter on the Struggle for Existence, Darwin writes "When we reflect on this struggle, we may console ourselves with the full belief that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.