284 DARWINISM AND THE PROBLEMS OF LIFE 



adaptive colouringf. Most animals need their colouring 

 as a protection against their enemies, but in this case we 

 have organisms that are coloured for the purpose of 

 getting eaten. However, the life-story of the animals 

 enables us to understand this remarkable adaptation. 



We will not explore any further the interesting field 

 of parasitism. We have examined the various forms of 

 parasites and their habits so closely, because they make 

 it so clear that there are no insuperable limits fixed to 

 the variations of animals. As a matter of fact, we dare 

 not say that natural selection could not give wings to a 

 horse. If there were some necessity and sufficient time 

 for such a transformation of the horse, the required 

 variations would certainly not fail. The creation of a 

 pegasus is not more wonderful than the transformation 

 of a spider into a tapeworm-like animal. Natural selec- 

 tion is omnipotent in the creation of forms of life. It 

 is restricted by no limit that lies in the nature of things. 



There is a certain resemblance to parasitism in a 

 phenomenon that leads us on to the coelenterata. This 

 is symbiosis, or community of life. 



When we put water and plants from a pond into an 

 aquarium, we often notice on the glass, some time after- 

 wards, a green tube, as thin as a needle, with long 

 threads hanging from the end of it. If a water-flea 

 touches one of these threads, it remains sticking to it, 

 and is conveyed into the tube, where it disappears. 



This small, tubular creature is the green water- 

 polyp, one of the few fresh-water representatives of 

 the coelenterata, which are found in many brightly 



