DARWINISM DEFENDED. 173 



atively increased. The individuals which survive by these 

 aids may then later, through inter-crossing, exchange their 

 advantages and so lead to the production of a mean type 

 that shows a slight advance over earlier conditions in all 

 three organs. Mammals can protect themselves from flies 

 in various ways, either by a thick fur (bears), by reflexive 

 twitchings of the skin muscles (horse), by tails with tufted 

 ends (many hoofed animals), by a long neck which can bend 

 sideways so that the animal can reach any part of its body 

 as far as the hips (guanaco, stag) or finally by eyelids and 

 long movable ears, which restrain the flies from the easily 

 injured eyes. These means of protection from insects play 

 an important role in determining the habitus of many mam- 

 mals, and permit the conclusion that selection has been 

 instrumental in producing this habitus. Here, also, it is 

 possible for several of these means to be possessed at once 

 by the same animal, as in the case of the guanaco with its 

 long hair and elongate neck. It is absolutely necessary for 

 molluscs that live between tide-lines to have some means of 

 resisting the force of the surf. Many species possess this 

 means in their small size which allows them to crawl into 

 crevices and cracks in the rocks, but most of them have 

 developed a strong pedal sucker and a low roof-like uncoiled 

 shell which presses close to the rock surface and over which 

 the water flows without exerting any strong lateral pres- 

 sure. This is the case, for example, with Patella, Fissurella, 

 Chiton, Concholepas, Siphonaria, Gadinia, Calyptra, and 

 others. All three of these means of safety can come into 

 play simultaneously in selection, but it suffices when any 

 given individual possesses any one of these means in suffi- 

 cient degree of development. 



( T shall note here several other examples which show 

 how related species reach the same advantage in the strug- 

 gle through different means, for we can assume from these 

 facts that also the individuals of the same species often 



