SUCCESSION OF CHARACTERS. 267 



limits, between the embryonic stages of growth of higher 

 animals and the permanent characters of others of an in- 

 ferior grade. Now, the fact mentioned above enables us 

 to mark with precision the limits within which these 

 relations may be traced. As eggs, in their primitive con- 

 dition, animals do not differ one from the other ; but as 

 soon as the embryo has begun to show any characteristic 

 features, it presents such peculiarities as distinguish its 

 branch. It cannot, therefore, be said that any animal 

 passes through phases of development which are not in- 

 cluded within the limits of its own branch. No Verte- 

 brate is, or resembles at any time, an Articulate ; no 

 Articulate a Mollusk ; no Mollusk a Radiate; and vice 

 versa. Whatever correlations between the young of 

 higher animals and the perfect condition of inferior ones 

 may be traced, they are always limited to representatives 

 of the same branch ; for instance, Mammalia and Birds, in 

 their earlier development, exhibit certain features of the 

 lower classes of Vertebrates, such as the Reptiles or 

 Fishes ; Insects recall the Worms in some of their earlier 

 stages of growth, etc., but even this statement requires 

 qualifications, to which we shall have occasion to refer here- 

 after. However, this much is already evident, that no higher 

 animal passes through phases of development recalling all 

 the lower types of the animal kingdom, but only such as 

 belong to its own branch. What has been said of the 

 infusorial character of young embryos of Worms, Mol- 

 lusks, and Radiates, can no longer stand before a serious 

 criticism, because, in the first place, the animals generally 

 called Infusoria cannot themselves be considered as a 

 natural class; and in the second place, those to which a 

 reference is made in this connexion are themselves free- 

 moving embryos. 1 



1 See above, Chap. I, Sect. 18. 



