192 THE GENESIS OF SPECIES. [CHAP. 



mollusca, but it is difficult to see why the shell 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 the successive chambers of the orthoceratidae. 

 Nor 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." 1 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 may 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 this process much 

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

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

 reached by the Eurypterida and Trilobites of the palaeozoic 

 strata ; and annelids, in all probability formed mainly like 

 those of the present day, abounded during the deposition 

 of the 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 Mr. Spencer's mechanical explanation. 



On the theory of evolution most in favour, the first 

 Yertebrata were aquatic. Now, as natation is generally 



1 " Principles of Biology, " voL iL p. 105. 



