192 THE GENESIS OF SPECIES. [Chap. 



iiiollusca, but it is difficult to see wliy the sliell segments 

 of chiton are not such homologues because the segmenta- 

 tion is superficial. The external processes of eolis, doris, 

 &c., are good examples of serial homology, as also are 

 plainly tlie successive chambers of the orthoceratidie. 

 Xor are parts of a series less serial because arranged 

 spirally, as in most gasteropods. Mr. Spencer observes 

 of the molluscous as of the vertebrate animal, " You 

 cannot cut it into transverse slices, each of which contains 

 a digestive organ, a respiratory organ, a reproductive 

 organ, &c." ^ But the same may be said of every single 

 arthropod and annelid, if it be meant that all these organs 

 are not contained in every possible slice. While if it be 

 meant that parts of all such organs are contained in 

 certain slices, then some of the mollusca mav also be 

 included. 



Another objection to Mr. Spencer's speculation is de- 

 rived from considerations which have already been stated, 

 as to past time. For if the annulose animals have been 

 formed by aggregation, we ought to find tliis process much 

 less perfect in tlie oldest form. But a complete develop- 

 ment, such as already obtains in the lobster, &c., was 

 reached by the Eurypterida and Trilobites of the pala30zoic 

 strata ; and annelids, in all probability formed mainly like 

 those of the present day, abounded during the deposition 

 of tlie oldest fossiliferous rocks. 



Thirdly, and lastly, as regards such serial homology as 

 is exemplified by the backbone of man, there are also 

 several objections to ^fr. Spencer's mechanical explanation. 



On the theory of evolution most in favour, the first 

 Yertebrata were aquatic. Now, as natation is generally 



^ "Piiiaiples of Biology,"^ vol. ii. p. 105. 



