THE FACTORS OF EVOLUTION 199 



region of the Amazon Kiver feed the common green parrot with 

 the fat of certain cat-fishes and thus produce birds with brilliant 

 red and yellow feathers. Finally, it is well known that horse- 

 dealers frequently dose horses before a sale with small quantities 

 of arsenic, as a result of which the body fills out and the hair 

 assumes a smooth and silky appearance. 



I must not omit to mention here that a change of external 

 conditions does by no means affect 'all organisms in the same 

 manner, but that rather many stimuli which effect radical changes 

 in one animal species frequently pass by others for considerable 

 periods without producing the slightest effect. An instance will 

 make this clear. Towards the middle of last century the 

 delicate, branched trees of a polyp, Cordylophora lacustris, were 

 only known on the European and North American coastlands- 

 Suddenly, however, the little polyp began to wander up the river 

 into the interior, and now we find them in many parts of Germany 

 inhabiting rivers and inland lakes. In many towns Cordylophora 

 makes itself most unpleasantly noticeable by invading the water- 

 pipes, where it proceeds to grow luxuriantly, and thereby finally 

 stops them up. The life-history of this polyp forms one of the 

 few instructive cases of a typical marine species changing 

 within a short time and almost under our very eyes into a fresh- 

 wateranimal. But in spite of the greatly changed conditions of life 

 the organism of the animal has hitherto been in nowise modified. 



If we observe the cycle of evolution of an animal or plant 

 as a whole we see how numerous different stages follow upon 

 one another in law-governed succession. As the hand of a clock 

 must traverse one hour after another, and can only indicate the 

 twelfth hour after having previously indicated the eleventh, so 

 in the organic world the succession of stages appears to be 

 unalterable. The embryo develops into a child, the child 

 becomes a boy, the boy a man, and only he may reproduce a 

 similar offspring which in his turn must travel the same cycle. 

 This regularity is manifest throughout the whole of Nature, 

 and it does not, therefore, cause surprise that the mind of man 

 conceives it as an iron necessity. According to a famous botanist 

 ' each stage of development generates the next in succession ; 

 it operates continuously ; there is no single stage which may be 

 omitted. Each differentiation is the condition of the next. 



