INTRODUCTION 17 



in the fertilised ovum, there is no visible evidence of any of 

 the organs and characters of later life. These arise by the 

 multiplication of cells and differentiation of tissues. We 

 must consider, therefore, whether their development affords 

 evidence of the manner and causes of their evolution. 



Now the embryos of the higher vertebrates all exhibit 

 certain characters in common, in the presence of gill-arches 

 and gill-slits, and in the origin of the limbs as bud-like 

 outgrowths. The great embryologist of the beginning of 

 this century, Von Baer, whose studies were directed princi- 

 pally to the higher vertebrates, formulated the generalisation 

 that animals of different classes resembled each other closely 

 in the earlier stages of their development, and diverged 

 more and more as they progressed towards their final form. 

 This remains true of the higher classes of vertebrates, reptiles, 

 birds, and mammals. When the doctrine of evolution 

 became paramount, and it was seen that the comparative 

 anatomy of the higher vertebrates obviously pointed to 

 their common derivation from ancestors which were 

 essentially fishes, the resemblance in the structure of the 

 embryos was attributed to the retention in these embryos 

 of the essential characters of the fish. The generalisation 

 of Von Baer was therefore changed into another, to wit, 

 that in development the individual passed successively 

 through the stages of its ancestors to arrive at its present final 

 condition. Haeckel gave great publicity to this doctrine, 

 calling it the biogenetic law, and formulating it in the terms 

 that ontogeny, or the development of the individual, is a 

 repetition of phylogeny, or the evolution of the race. The 

 late Professor Milnes Marshall still further popularised and 

 established the principle by embodying it in another phrase, 

 namely, that the individual in development was compelled 

 to climb its own genealogical tree. 



A more comprehensive and more accurate study of the 



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