202 LIFE ON THE EARTH. 



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 have de- 

 scended from one prototype. But analogy may be 

 a deceitful guide. Nevertheless all living things 

 have much in common, in their chemical composition, 

 their germinal vesicles, their cellular structure, and 

 their laws of growth and reproduction. We see this 

 even in so trifling a circumstance 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. 

 Therefore I should infer from analogy that probably 

 all the organic beings which have ever lived on this 

 earth have descended from some one primordial 

 form into which life was first breathed. 



* As all the living forms of life are the lineal de- 

 scendants of those which lived long before the Si- 

 lurian epoch, we may feel certain that the ordinary 

 succession by generation has never once been broken, 

 and that no cataclysm has desolated the whole world. 

 Hence we may look with some confidence to a secure 

 future of equally inappreciable length. And as 

 natural selection works solely by and for the good 

 of each being, all corporeal and mental endowments 

 will tend to progress toward perfection. 



