This idea of the influence of the external world as a factor inducing changes in the organism has come, however, to be associated especially with the name of Geoffroy Saint-Hilaire, whose period of activity, although overlapping, came after that of Lamarck. The central idea of Geoffroy’s view was that species of animals and plants undergo change as the environment changes; and it is important to note, in passing, that he did not suppose that these changes were always for the benefit of the individual, _i.e._ they were not always adaptive. If they were not, the forms became extinct. So long as the conditions remain constant, the species remains constant; and he found an answer in this to Cuvier’s argument, in respect to the similarity between the animals living at present in Egypt and those discovered embalmed along with mummies at least two thousand years old. Geoffroy Saint-Hilaire said, that since the climatic conditions of Egypt had remained exactly the same during all these years, the animals of Egypt would also have remained unchanged. Geoffroy’s views were largely influenced by his studies in systematic zoology and by his conception of a unity of plan running through the entire animal kingdom. His study of embryology and paleontology had led him to believe that present forms have descended from other organisms living in the past, and in this connection his discovery of teeth in the jaws of the embryo of the baleen whale and also his discovery of the embryonic dental ridges in the upper and in the lower jaws of birds, were used with effect in supporting the theory of change or evolution. Lastly, his remarkable work in the study of abnormal forms prepared the way for his conception of sudden and great changes, which he believed organisms capable of undergoing. He went so far in fact, in one instance, as to suppose that it was not impossible that a bird might have issued fully equipped from the egg of a crocodile. Such an extreme statement, which seems to us nowadays only laughable, need not prejudice us against the more moderate parts of his speculation. His study of the fossil gavials found near Caen led him to believe that they are quite distinct from living crocodiles. He asked whether these old forms may not represent a link in the chain that connects, without interruption, the older inhabitants of the earth with animals living at the present time. Without positively affirming that this is the case, he did not hesitate to state that a transformation of this sort seemed possible to him. He said: “I think that the process of respiration constitutes an acquirement so important in the ‘disposition’ of the forms of animals, that it is not at all necessary to suppose that the surrounding respiratory gases become modified quickly and in large amount in order that the animal may become slowly modified. The prolonged action of time would ordinarily suffice, but if combined with a cataclysm, the result would be so much the better.”