1883.] 265 [Hyatt. 



ciate all forms which have similar embryos, and dissociate in 

 classification all forms having different embryos. As a matter of 

 experience, the surest guides of affinity are the adult gradations of 

 forms. These show that the Nautiloidea and Ammonoidea with 

 comparatively distinct embryos are nevertheless closer related than 

 the Belemnoidea and Ammonoidea which have precisely similar 

 embryos, and Sepioidea and Belemnoidea which have very distinct 

 embryos must also closely be affiliated. 



The embryos of all these must have been precisely similar at 

 their origin, but they afterwards became varied in the different 

 orders, and we cannot lay down any hard and fast rule by which 

 the embryo becomes an invariable criterion of affinity. We think 

 there is ample reason in the structures of these shells themselves 

 for the embryonic differences, and that it is possible to reconcile 

 them with the affinities indicated by the gradations observed 

 between the adults. These reasons which we have space only to 

 allude to here consists in a series of correlations which are plainly 

 apparent between the adult structures, and the habits of the 

 animals, and the tendencies which the habits have to change the 

 adult structures, and then by the action of the law of concentra- 

 tion in development to change even the embryos, either quickly 

 in time when the habits are widely changed, or more slowly when 

 they vary but slightly with the progress of time. The evolution 

 is a purely mechanical problem in which the action of the habitat 

 is the working agent of all the major changes ; first acting upon 

 the adult stages as a rule, and then through heredity upon the 

 earlier stages in successive generations. Thus in the open fields 

 of the periods of their origin they expanded into their different 

 habitats, varying to accomplish this purpose with great rapidity, 

 but once in their appropriate habitat inducements to change or 

 open fields became rarer, and we get as a result comparative 

 invariability. As time rolled on and the earth became more 

 crowded, the variability was reduced to less and less important 

 structural changes, except in the retrogressive types. These 

 exceptions are our best proofs of the action of the habitat. The 

 changes in these retrograde forms are again remarkable for the 

 rapidity with which they take place, and all of these types can be 

 shown to have occupied free fields where they met with new 

 conditions, and to have changed their habits and structures rapidly 

 to accord with these new conditions. 



