322 ADAPTIVENESS AND PURPOSIVENESS 



eggs. The young marsupials are born prematurely and can- 

 not even suck ; the mother places them in her external pocket 

 of skin and has a special arrangement for forcing the milk 

 into their mouth. They meet this, so to speak, with a special 

 adaptation that prevents the milk going down the wrong way. 



There are also inter-organismal adaptations between crea- 

 tures of different kinds, of which the crovniing examples 

 are to be found in the way certain flowers and certain in- 

 sects are suited for making the best of one another. Very 

 striking also are the numerous mutually helpful associations 

 which have been established — partnerships, commensalism, 

 and symbiosis, in which there is sometimes two-sided adapta- 

 tion. The case is repulsive, but the parasite is often adapted 

 to its drifting life of ease and to making much of its host, 

 which, in turn, is often adapted so that it hardly suffers 

 at all from its guest. In the mimetic resemblance of one 

 creature to another there is again adaptation, often of almost 

 incredible subtlety. 



In Indo-Pacific crabs of the genus Melia a delicate sea- 

 anemone is often carried on the forceps, and probably serves 

 to paralyse the crab's prey with its batteries of stinging 

 cells. It is quaintly suggestive of a tool, and its occasional 

 absence shows that it is not vitally necessary. But the part- 

 nership or commensalism is probably of very old standing, 

 since the denticles of the forceps are elongated into needles 

 which are adaptively suited to keeping a firm grip of the 

 ' tool \ 



We are accustomed to the idea of adaptations, but perhaps 

 we are not sufficiently appreciative of their nicety. When 

 winter sets in, the North American ruffed gi^ouse puts on 

 snow-shoes — a row of projecting plates on each side of each 

 toe so that the bird can tread on the loose snow without 



