182 THE GENESIS OF SPECIES. [CHAP. 



ganic structures. With regard to organisms, however, the 

 wonderful Acanthometraa and the Polycystina may be men- 

 tioned as presenting complexities of form which can hardly 

 be thought to be due to other than internal causes. The 

 same may be said of the great group of Echinoderms, with 

 their amazing variety of component parts. If, then, internal 

 forces can so build up the most varied structures, they are 

 surely capable of producing the serial, lateral, and vertical 

 symmetries which higher animal forms exhibit. Mr. Spen- 

 cer is the more bound to admit this, inasmuch as in his doc- 

 trine of " physiological units " he maintains that these or- 

 ganic atoms of his have an innate power of building up and 

 evolving the whole and perfect animal from which they 

 were in each case derived. To build up and evolve the 

 various symmetries here spoken of is not one whit more 

 mysterious. Directly to refute Mr. Spencer's assertion, 

 however, would require the bringing forward of examples 

 of organisms which are ill-adapted to their positions, and 

 out of harmony with their surroundings a difficult task 

 indeed. 8 



Secondly, as regards the last-mentioned author's expla- 

 nation of such serial homology as exists in the centipede and 

 its allies, the very groundwork is open to objection. Mul- 

 tiplication by spontaneous fission seems from some recent 



8 Just as Buffon's superfluous lament over the unfortunate organiza- 

 tion of the sloth has been shown, by the increase of our knowledge, to 

 have been uncalled for and absurd, so other supposed instances of non- 

 adaptation will, no doubt, similarly disappear. Mr. Darwin, in his " Ori- 

 gin of Species," 5th edition, p. 220, speaks of a woodpecker (Colaples 

 campestris) as having an organization quite at variance with its habits, 

 and as never climbing a tree, though possessed of the special arboreal 

 structure of other woodpeckers. It now appears, however, from the ob- 

 servations of Mr. W. H. Hudson, C. M. Z. S., that its habits are in har- 

 mony with its structure. See Mr. Hudson's third letter to the Zoological 

 Society, published in the Proceedings of that Society for March 24, 1 870, 

 p. 159. 



