CONCLUSION. 469 



fcage is gained by believing that new forms are suddenly devel- 

 oped in an inexplicable manner from old and widely different 

 forms, over the old belief in the creation of species from the 

 dust of the earth. 



It may be asked how far I extend the doctrine of the 

 modification of species. The question is difficult to answer, 

 because the more distinct the forms are which we consider, 

 by so much the arguments in favor of community of descent 

 become fewer in number and less in force. But some argu- 

 ments of the greatest weight extend very far. All the 

 members of whole classes are connected together by a chain 

 of affinities, and all can be classed on the same principle, in 

 groups subordinate to groups. Fossil remains sometimes 

 tend to fill up very wide intervals between existing orders. 



Organs in a rudimentary condition plainly show that an 

 early progenitor had the organ in a fully developed condi- 

 tion, and this in some cases implies an enormous amount of 

 modification in the descendants. Throughout whole classes 

 various structures are formed on the same pattern, and at a 

 very early age the embryos 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 further, 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 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, ex- 

 cepting perhaps some of the very lowest, sexual reproduc- 

 tion 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 . age- 

 table kingdoms — certain low forms are so far intermediate 

 in character that naturalists have disputed to which l l. j- 

 dom they should be referred. As Professor Asa Gray has 

 remarked, "the spores andQtUer reurod active bodies of -nany 



