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.

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