Both treat of domesticated animals, but in very different ways. Darwin uses them as the practical illustration of selection, and argues that if man by selection can make such forms, Nature can make her species by the same means. Wallace disposes of the argument that the reversion of domesticated varieties to the wild form is a proof of the permanent distinctness of species, by showing in some detail that the former are “abnormal, irregular, artificial.” Neither of them draws any distinction between instinct and other qualities, but assumes that the former is, like the latter, operated upon by natural selection. Wallace makes a special point of protective resemblances in the colours of insects, etc. The important principle of “divergence of character,” and the relatively unimportant one of “sexual selection,” are both clearly explained by Darwin. Neither writer speaks of the direct effect of external conditions--except as a cause of plasticity by Darwin--or the inherited effects of use and disuse. Lamarck is mentioned only to be dismissed by Wallace. The evolution of the giraffe’s long neck is explained by Wallace on the principle of natural selection, which is contrasted with Lamarck’s original explanation of the same character. This contrast, which has been so often drawn, was therefore originally contained in the first public statement of natural selection. As has been indicated above, Darwin suggested a cause of variation in the direct effect of changed external conditions on the reproductive system. In comparing the two essays it is not unnatural to conclude, as Professor Osborn has done (“From the Greeks to Darwin,” 1894, p. 245), that the two writers held different views upon the material utilised by natural selection in the production of new species, Darwin relying upon the usual slight differences which separate individuals and upon variations in single characters, Wallace upon fully formed varieties--viz. individuals which departed conspicuously from the type of the species, and which may exist singly or in considerable numbers side by side with the parent form. Professor Osborn’s actual words are as follows:-- “Darwin dwells upon _variations in single characters_, as taken hold of by Selection; Wallace mentions variations, but dwells upon _full-formed varieties_, as favourably or unfavourably adapted. It is perfectly clear that with Darwin the struggle is so intense that the chance of survival of each individual turns upon a single and even slight variation. With Wallace, varieties are already presupposed by causes which he does not discuss, a change in the environment occurs, and those varieties which happen to be adapted to it survive. There is really a wide gap between these two statements and applications of the theory.”