CHAPTEE VI. 



THE ARGUMENTS FROM MORPHOLOGY. 



133. LEAVING out of consideration the parallelism oi 

 development which characterizes organisms belonging to each 

 group, that community of plan which exists among them 

 when they are mature, is extremely remarkable and extremely 

 suggestive. As before shown ( 103), neither the supposition 

 that these combinations of attributes which unite classes are 

 fortuitous, nor the supposition that no other combinations 

 were practicable, nor the supposition of adherence to pre- 

 determined typical plans, suffices to explain the facts. An 

 instance will best prepare the reader for seeing the true 

 meaning of these fundamental likenesses. 



Under the immensely- varied forms of insects, greatly elon- 

 gated like the dragon-fly, or contracted in shape like the 

 lady-bird, winged like the butterfly, or wingless like the 

 flea, we find this character in common there are primarily 

 twenty segments. These segments may be distinctly marked, 

 or they may be so fused as to make it difficult to find the 

 divisions between them. This is not all. It has been 

 shown that the same number of segments is possessed by all 

 the Cmstacea. The highly-consolidated crab, and the squilla 

 with its long, loosely-jointed divisions, are composed of the 

 same number of somites. Though, in the higher crustaceans, 

 some of these successive indurated rings, forming the exo- 

 skeleton, are never more than partially marked off from each 



