CHAP. XV.] CONCLUSION. 399 



closely resemble each other. Therefore I cannot doubt that the 

 theory of descent with modification embraces all the members of 

 the same great class or kingdom. I believe that animals are 

 descended from at most only four or five progenitors, and plants 

 from an equal or lesser number. 



Analogy would lead me one step farther, namely, to the belief 

 that all animals and plants are descended from some one proto- 

 type. 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 injurious 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 repro- 

 " ductive bodies of many of the lower algae may claim to have first 

 "a characteristically animal, and then an unequivocally vegetable 

 " existence." Therefore, 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 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 not it be 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 groat kingdom, 

 such as the Vertebrata, 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 



