72 Miscellaneous. 



organs, and specific characters ; and different results arise from 

 their unequal development. " Thus, two series of histogenic phae- 

 nomena of the same kind, in two different animals, or two series of 

 such phsenomena of different kinds in the same animal, may offer at 

 first a certain number of corresponding terms ; but at a period more 

 or less advanced, these terms will cease to be analogous, and in 

 general the divergence will continue to increase as they approach the 

 final result." In this manner, by an arrest of development of dif- 

 ferent tissues or organs, at different periods, one animal becomes 

 widely different from another, and there v*dll be an analogy between 

 a finished tissue of what we designate an inferior grade, and one 

 of the steps in the development of a superior. " By the appli- 

 cations of these principles, we explain the concordance between the 

 permanent forms of certain animals, and the transitor)'' state of the 

 embryo in other species whose metamorphic career is longer con- 

 tinued." In a natural group, the species, with a short period of 

 development, start upon the same route which those have followed 

 that have left them on the road. The animals, therefore, whose 

 embryogenic career is of different lengths, constitute a number of 

 separate series, more or less closely related, according as their dif- 

 ferences in zoogenic progress begin more early and are wider in 

 character. The progress of metamorphosis produces generally a 

 higher and higher grade. But there are instances of a degradation 

 from the same source, as with the Lernseas, which, in a transitory 

 state, have a rank corresponding with the Cyclops, though after- 

 wards so inferior in character. 



In view of these facts, we perceive the foundation of the homo- 

 logies and analogies which have been observed between animals 

 and groups of animals*. We comprehend how the secondary modi- 

 fications running through one group are rejieated in another series. 

 A natural group, says Milne-Edwards, is a reunion of all the deriva- 

 tives of the same type ; a primary division includes all derived from 

 the same primary type, or i)lan of development ; and a secondary 

 division, those from a secondary type. There are types of the first, 

 second, third, fourth, &c., orders, and groups corresponding to each. 

 Moreover each group may contain several natural series, parallel pr 

 otherwise, and more or less elevated in rank. 



The importance of embryogeny as a means of distinctions is at 

 once shown by the embryonary forms of animals of the four grand 

 divisions of the animal kingdom, as long since laid down by Baer. 

 In the ovum of the vertebrate animal, the first step is the formation 

 of the medial depression which divides the central portion of the 

 blastoderm into two symmetrical halves, and corresponds to the ver- 

 tebral column and its adjuncts. There is nothing similar in the In- 

 vertebrata. Thus the very first point observed in the embryo of a 

 vertebrate animal is that which is the dominant characteristic in 

 this whole division. Other peculiarities are pointed out in the me- 



* Milne- Ed wards uses the term direct affinity for tlie immediate relations 

 of species ; collateral affinity ^ for the relations of parallel series. 



