The Origin of Species 



become fewer in number and less in force. But 

 some arguments 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 some- 

 times 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 condition, and this in some cases 

 implies an enormous amount of modification in 

 the descendants. Throughout whole classes 

 various structures are formed on the same pat- 

 tern, 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 arc de- 

 scended from at most only four or five progeni- 

 tors, 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 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 

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