“vr EVOLUTION IN BIOLOGY 219 
pllowing 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 
. sk eleton, 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 
| e stricted within the limits of one type of organi- 
vtion ; and, if it is further recollected that the 
resemblance between the permanent lower form 
vand 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 Geoffroy 
Saint-Hilaire and Serres, was no doubt an over- 
statement of the case. It is not true, for example, 
ithat 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 
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