Chap. XV.] CONCLUSION. 299 



five progenitors, and plants from an equal or lesser 

 number. 



rAnalogy would lead me one step farther, namely, to 

 the belief that all animals and plants are descended from 

 some one prototype?^ But analogy may be a deceitful 

 guide. Nevertheless all living things have much in 

 common, in their chemical composition, their cellular 

 structure, their laws of growth, and their liability to in- 

 jurious influences. T We see this even in so trifling a 

 fact as that the same poison often similarly aSects 

 plants and animals ; or that the poison secreted by the 

 gall-fly produces monstrous growths on the wild rose or 

 oak-tree. With all organic beings, excepting perhaps 

 some of the very lowest, sexual rex^roduction seems to 

 be essentially similarj With all, as far as is at present 

 known, the germinal vesicle is the same; so that all 

 organisms start from a common origin. If we look even 

 to the two main divisions — namely, to the animal and 

 vegetable kingdoms — certain low forms are so far inter- 

 mediate in character that naturalists have disputed to 

 which kingdom they should be referred. As Professor 

 Asa Gray has remarked, " the spores and other repro- 

 " ductive bodies of many of the lower algse may claim 

 " to have first a characteristically animal, and then an 

 " unequivocally vegetable existence." Therefore, on the 

 principle of natural selection with divergence of char- 

 acter, it does not seem incredible that, from some such 

 low and intermediate form, both animals and plants 

 may have been developed; and, if we admit this, we 

 must likewise admit that all the organic beings which 

 have ever lived on this earth may be descended from 

 some one primordial form. But this inference is chiefly 

 grounded on analogy, and it is immaterial whether or 



