THE LAW OF EVOLUTION 



we ascend the vegetal scale, we find the ferns and 

 lichens decidedly more heterogeneous than the 

 algae ; and as we meet with endogens and exo- 

 gens, we find the increasing heterogeneity ac- 

 companied by a definiteness and coherence of 

 structure that is ever more and more conspicu- 

 ous. Going up the animal scale, we find the 

 annulosa, on the whole, much more heterogene- 

 ous, definite, and coherent than the mollusca ; 

 while the vertebrata, on the whole, exhibit these 

 characteristics more strikingly than either of 

 the other sub-kingdoms. The relatively homo- 

 geneous and unintegrated polyps are ranked be- 

 low all of these. Within each group the same 

 principle of classification is universally followed. 

 Contrast the centipede, whose multitudinous 

 segments are almost literally copies of each 

 other, or the earthworm, which may be severed 

 in the middle and yet live, with the highly 

 differentiated and integrated hive-bee, spider, 

 or crab. Compare the definite and symmetrical 

 contour of the cuttlefish, which is the highest 

 of the mollusca, with the unshapely outline of 

 the molluscoid ascidians. Or, to cite cases from 

 the two extremes of the animal scale, consider 

 first the complicated mammal, whose growth 

 from the embryo we have lately contemplated ; 

 and then turn to the hydra, or fresh-water polyp, 

 which is a mere bag of organized matter, di- 

 gesting with its inner surface and respiring with 



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