CRITICISM OF THEORY OF PARALLELISM 121 



development from which it followed that evolution, whether 

 conceived as an ideal or as an historical process, could 

 take place only along one line, could be only progressive 

 or regressive. Not all the supporters of the theory of 

 parallelism held these extreme views, but conclusions of 

 this kind were natural and logical enough. 



Von Baer had soon found in the course of his embryo- 

 logical studies that the facts did not at all fit in with the 

 doctrine of parallelism ; the developing chick, for example, 

 was at a very early stage demonstrably a Vertebrate, and did 

 not recapitulate in its early stages the organisation of a 

 polyp, a worm or a mollusc. He had published his doubts in 

 1823, but his final confutation of the theory of parallelism is 

 found in this Scholion. 



If it were true, he says, that the essential thing in the 

 development of an animal is this repetition of lower organisa- 

 tions, then certain deductions could be drawn, which one 

 would expect to find confirmed in Nature. The first deduction 

 would be that no structures should appear in the embryo of 

 the higher animals that are not found in the lower animals. 

 But this is not confirmed by the facts no adult among the 

 lower animals, for instance, has a yolk-sac like that of the 

 chick embryo. Again, if the law of parallelism were true, 

 the mammalian embryo would have to repeat the organisa- 

 tion of, among other groups, insects and birds. But the 

 embryo in utero is surrounded by fluid and cannot possibly 

 breathe free air, so it cannot possibly repeat the structure of 

 either insects or birds, which are pre-eminently air-organisms. 

 Generally speaking, indeed, we find in all the higher embryos 

 special structures which adapt them to the very special con- 

 ditions of their development, and these we never find as 

 permanent structures in the lower animals. The supporters of 

 the theory of parallelism might, however, admit the existence 

 of such special embryonic organs without greatly prejudicing 

 their case, for these temporary organs stand to some extent 

 outside the scope of the theory. 



But they would have to face a second and more important 

 deduction from their views, namely, that the higher animals 

 should repeat at every stage of their development the whole 

 organisation of some lower animal, and not merely agree 



