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Before proceeding to the post-Darwinian investigations that have done so much to amplify the account of natural evolution, let us consider the contrasted explanation given by Lamarck and his followers.  As we have stated earlier, Lamarckianism is the name given to the doctrine that modifications other than those due to congenital factors may enter into the heritage of a species, and may add themselves to those already combined as the peculiar characteristics of a particular species.  Let us take the giraffe and its long neck as a concrete example.  The great length of this part is obviously an adaptive character, enabling the animal to browse upon the softer leafy shoots of shrubs and trees.  The vertebral column of the neck comprises just the same number of bones that are present in the short-necked relatives of this form, so that we are justified in accepting as a fact the evolution of the giraffe’s long neck by the lengthening of each one of originally shorter vertebrae.  The Lamarckian explanation of this fact would be that the earliest forms in the ancestry of the giraffe as such stretched their necks as they fed, and that this peculiar function with its correlated structural modification became habitual.  The slight increase brought about by any single individual would be inherited and transmitted to the giraffes of the next generation; in other words, an individually acquired character would be inherited.  The young giraffes of this next generation would then begin, not where their parents did, but from an advanced condition.  Thus, by continued stretching of the neck and by continued transmission of the elongated condition, the great length of this part of the body in the modern giraffe would be attained.

The explanation of natural selection would be quite different.  The Darwinian would say that all the young giraffes of any one generation would vary with respect to the length of the neck.  Those with longer necks would have a slight advantage over their fellows in the extended sphere of their grazing territory.  Being better nourished than the others, they would be stronger and so they would be more able to escape from their flesh-eating foes, like the lion.  For the reason that their variation would be congenital and therefore already transmissible, their offspring would vary about the advanced condition, and further selection of the longer necked individuals would lead to the modern result.

The Lamarckian explanation encounters one grave difficulty which is not met by the second one, in so far as it demands some method by which a bodily change may be introduced into the stream of inheritance.  So far, this difficulty has not been overcome, and the present verdict of science is that the transmission of characters acquired as the result of other than congenital factors is not proved.  It would be unscientific to say that it cannot be proved in the future, but there are good a priori grounds