VI EVOLUTION IN BIOLOGY 219 



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. Nothwithstanding 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. tit. 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 organi- 

 sation ; 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 im- 

 proved 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- 

 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 

 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 



