310 EVOLUTION IN BIOLOGY. 



"From tLese lowest Vertebrata to the highest, and to the 

 highest forms among these, the comparison between the embryonic 

 conditions of the higher animals and the adult states of the lower 

 can be more completely and thoroughly instituted than if the 

 survey is extended to the Invertebrata, inasmuch as the latter are 

 in many respects constructed upon an altogether too dissimilar 

 type ; indeed they often differ from one another far more than the 

 lowest vertebrate does from the highest mammal ; yet the following 

 pages will show that the comparison may also be extended to them 

 with interest. In fact, there is a period when, as Aristotle long 

 ago said, the embryo of the highest animal has the form of a mere 

 worm; and, devoid of internal and external organisation, is merely 

 an almost structureless lump of polype-substance. Notwithstanding 

 the origin of organs, it still for a certain time, by reason of its want 

 of an internal bony skeleton, remains worm and mollusk, and only 

 later enters into the series of the Vertebrata, although traces of the 

 vertebral column even in the earliest periods testify its claim to a 

 place in that series." Op. cit. pp. 4, 5. 



If Meckel's proposition is so far qualified, that tbe 

 comparison of adult with embryonic forms is restricted 

 within the limits of one type of organisation ; and, if it 

 is further recollected that the resemblance between the 

 permanent lower form and the embryonic stage of a 

 higher form is not special but general, it is in entire 

 accordance with modern embryology ; although there is 

 no branch of biology which has grown so largely, and 

 improved its methods so much, since Meckel's time, as 

 this. In its original form, the doctrine of " arrest of de- 

 velopment," as advocated by Geoffroy Saint-IIilaire and 

 Serres, was no doubt an over-statement of the case. It 

 is not true, for example, that a fish is a reptile arrested 

 in its development, or that a reptile was ever a fish ; but 

 it is true that the reptile embryo, at one stage of its 



