RECAPITULATION AND CONCLUSION 523 



ing to furtlier investigation, but little advantage is gained 

 by believing that new forms are suddenly developed in an 

 inexplicable manner from old and widely different forms, 

 over the old belief in the creation of species from the dust o£ 

 the earth. 



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

 fication of species. The question is difficult to answer^ be- 

 cause the more distinct the forms are which we consider, by 

 so much the arguments in favour of community of descent 

 become fewer in number and less in force. But some 

 argtiments 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 exist- 

 ing 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 farther, namely, to the 

 belief that all animals and plants are descended from some 

 one prototype. But analogy may be a deceitful guide. Never- 

 theless 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 se- 

 creted 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 



