THE SCIENCE OF EMBRYOLOGY 9 



Structures which of course are present and have a function in aduk fish 

 but disappear in the bird before the adult stage is reached. Fairly shortly- 

 after, the improvement of microscopes made it possible for von Baer to 

 show that an embryo never looks exactly like an adult of any kind. The 

 gill-sHts of a bird embryo are rather like those of a fish embryo, but only 

 remotely resemble those of an adult fish. 



As von Baer pointed out, the fact is that young stages of different species 

 resemble each other more than older stages do, but this does not mean that 

 the stages in the development of an animal repeat its evolutionary history. 

 However, in spite of his commonsense, tliis idea of 'recapitulation', as it 

 was called, was revived after Darwin had made evolution the centre of 

 biological fashion again. Its cliief exponent was Haeckel, and for some 

 time it was taken as the guiding principle in embryology. It was sometimes 

 argued that evolutionary change always occurs by new stages being added 

 on at the end of development, so that the advanced animal goes through 

 the embryonic stages of its ancestors, perhaps in an accelerated and short- 

 ened form, then goes on a step or two further. But it was eventually borne 

 in on embryologists that von Baer had been right (cf. de Beer 195 1). And 

 as they came to reflect on the causal mechanisms underlying embryonic 

 development, it became clear that it is only to be expected that evolution- 

 ary alterations are much more likely to affect the later stages of develop- 

 ment, when comparatively minor features are being formed, and to leave 

 intact the earher steps on which all the later stages must depend. As a 

 matter of fact, in their very earliest stages the embryos of different types 

 of animals are rather radically different. It is at an intermediate period, early 

 but not right at the beginning, that embryos are most alike; probably 

 because this is the time at which the basic structure of the animal is 

 being rapidly laid down, and it is very difficult for evolution to alter 

 anything at such a crucial period without throwing everything into 

 confusion. 



It is, moreover, not true that an evolutionary advance always involves 

 the addition of something new to the original course of development. In 

 general it consists rather in a modification of the later stages in develop- 

 ment than in an addition to them. And there are several instances in which 

 the evolutionary novelty has been produced by arresting development 

 at an earlier stage than previously, so that thejuvenile form of the ancestor 

 becomes the adult of the descendant. In some respects, this has probably 

 happened in the evolution of man; the human adult has many features 

 which remind one of the young of apes (e.g. in the large skull with the 

 sutures between the bones closing very late, the form of the teeth, the 

 hairlessness of the skin, etc.). It has been argued, with perhaps less plausi- 



