VIII.] HOMOLOGIES. 185 



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 

 moUusca 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, etc., was 

 reached by the Eurypterida and Trilobites of the palaeozoic 

 strata ; and annelids, probably 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 sev- 

 eral objections to Mr. Spencer's mechanical explanation. 



On the theory of evolution most in favor, the first Ver- 

 tebrata were aquatic. Now, as natation is generally effected 

 by repeated and vigorous lateral flexions of the body, we 

 ought to find the segmentation much more complete laterally 

 than on the dorsal and ventral aspects of the spinal column. 



