External and Internal Factors in Evolution 301 
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, z.e. they were not always adap- 
tive. If they were not, the forms became extinct. So long 
as the conditions remain constant, the species remains con- 
stant; 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 dis- 
covery 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 be- 
lieved 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 
