236 LAMARCK AND DARWIN 



morphosis " may now be interpreted literally, and no longer 

 figuratively merely (p. 439). 



Very great interest attaches to Darwin's treatment of 

 development, for post- Darwinian morphology was based to a 

 very large extent on the presumed relation between the 

 development of the individual and the evolution of the race. 

 Just as he kept clear of the notion of the scale of beings, so 

 he avoided the snare of the Meckel-Serres theory of recapitu- 

 lation, according to which the embryo of the highest animal, 

 man, during its development climbs the ladder upon the 

 rungs of which the whole animal series is distributed, in its 

 gradual progression from simplicity to complexity. The law 

 of development which he adopts is that of von Baer, which 

 states that development is essentially differentiation, and 

 that as a result embryos belonging to the same group resemble 

 one another the more the less advanced they are in develop- 

 ment. There can be little doubt that he was indebted 

 to von Baer for the idea, and in the later editions of the 

 Origin he acknowledges this by quoting the well-known 

 passage in which von Baer tells how he had two embryos in 

 spirit which he was unable to refer definitely to their 

 proper class among Vertebrates. 1 



Not only are embryos more alike than adults, because 

 less differentiated, but it is in points not directly connected 

 with the conditions of existence, not strictly adaptive, that 

 their resemblance is strongest (p. 440) think, for instance, of 

 the arrangement of aortic arches common to all vertebrate 

 embryos. Larval forms are to some extent exceptions 

 to this rule, for they are often specially adapted to their 

 particular mode of life, and convergence of structure may 

 accordingly result. All these facts require an explanation. 

 " How, then, can we explain these several facts in embryology 

 namely, the very general, but not universal, difference in 

 structure between the embryo and the adult of parts in the 

 same individual embryo, which ultimately become very 

 unlike and serve for different purposes, being at this early 

 period of growth alike of embryos of different species 

 within the same class, generally but not universally, 



1 In the ist ed. (p. 439), Darwin makes the curious mistake 

 of attributing this story to Agassiz. 



