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CHAPTER VI. 



THE ARGUMENTS FROM MORPHOLOGY. 



133. LEAVING out of consideration those parallelisms 

 among their modes of development which characterize 

 organisms belonging to each group, that community of plan 

 which exists among them when 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 supposi- 

 tion 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 

 elongated 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 

 seventeen segments.* These segments may be distinctly 



* Early in our friendship (about 1855) Prof. Huxley expressed to me his 

 conviction that all the higher articulate animals have twenty segments or 

 somites. That he adhered to this view in 1880, when his work on The 

 Crayfish was published, is shown by his analysis there given of the twenty 

 segments existing in this fluviatile crustacean ; and adhesion to it had been 

 previously shown in 1877, when his work on The Anatomy of Inveitebrated 

 Animals was published. On p. 398 of that work he writes : " In the abdo- 

 men there are, at most, eleven somites, none of which, in the adult, bear 

 an.bulatory limbs. Thus, assuming the existence of six somites in the head, 

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