Chap. XV. Conclusion. 425 



things have much in common, in their chemical composition, their 

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

 jurious influences. We see this even in so trifling a fact as that the 

 same poison often similarly affects 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 reproduction seems to be 

 essentially similar. 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 intermediate in character that naturalists have disputed 

 to which kingdom they should be referred. As Professor Asa Gray 

 has remarked, " the spores and other reproductive bodies of many 

 " of the lower alga? may claim to have first a characteristically 

 " animal, and then an unequivocally vegetable existence." There- 

 fore, on the principle of natural selection with divergence of 

 character, it does not seem incredible that, from some such low and 

 intermediate form, both animals and plants may have been de- 

 veloped ; 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 not 

 it he accepted. No doubt it is possible, as Mr. G. H. Lewes has 

 urged, that at the first commencement of life many different forms 

 were evolved ; but if so, we may conclude that only a very few 

 have left modified descendants. For, as I have recently remarked 

 in regard to the members of each great kingdom, such as the 

 Vertehrata, Articulata, &c, we have distinct evidence in their 

 embryological, homologous, and rudimentary structures, that 

 within each kingdom all the members are descended from a single 

 progenitor. • 



When the views advanced by me in this volume, and by Mr. 

 Wallace, or when analogous views on the origin of species are 

 generally admitted, we can dimly foresee that there will be a 

 considerable revolution in natural history. Systematists will be 

 able to pursue their labours as at present; but they will not be 

 incessantly haunted by the shadowy doubt whether this or that 

 iorm be a true species. This, I feel sure and I speak after experience, 

 will be no slight relief. The endless disputes whether or not some 

 fifty species of British brambles are good species will cease. Syste- 

 matists will have only to decide (not that this will be easy) whether 

 ar ±y form be sufficiently constant and distinct from other forms, 



