XL] EVOLUTION IN BIOLOGY. 303 



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 the 

 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 development," as advocated by Geoffrey 

 Saint-Hilaire and Serres, was no doubt an over-state- 

 ment 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 development, is an organism 

 which, if it had an independent existence, must be 

 classified among fishes; and all the organs of the 

 reptile pass, in the course of their development, through 

 conditions which are closely analogous to those which 

 are permanent in some fishes. 



4. That branch of biology which is termed Mor- 



