3o6 The Study of Animal Life part iv 



nerves which usually degenerate in each individual life- 

 time. 



The theory that many structures in animals are due to 

 the inherited results of use and disuse has this advantage, 

 that it suggests a primary cause of change, whereas the 

 other theory assumes the occurrence of favourable variations 

 and proceeds to show how they might be accumulated in 

 the course of natural selection, that is to say by a secondary 

 factor in evolution. 



When we find in a large number of entirely distinct 

 forms that the same habit of life is associated with the same 

 peculiarities, there is a likelihood that the habit is a direct 

 factor in evolving these. Thus sluggish and sedentary 

 animals in many different classes tend to develop skeletons 

 of lime, as in sponges, corals, sedentary worms, lamp-shells, 

 Echinoderms, barnacles, molluscs. Professor Lang has re- 

 cently made a careful study of sedentary creatures, and this 

 result at least is certain that the same peculiarity often occurs 

 in many different types with little in common except that 

 they are sedentary. But till one can show that sedentary life 

 necessarily involves for instance a skeleton of lime or some- 

 thing equivalent, we are still dealing only with probabilities. 



2. The Influence of Surroundings. — In ancient times 

 men saw the threads of their life passing through the hands 

 of three sister-fates — of. one who held the distaff, of another 

 who offered flowers, and of a third who bore the abhorred 

 shears of death. In Norseland the young child was visited by 

 three sister Norns, who brought characteristic gifts of past, 

 present, and future, which ruled the life as surely as did 

 the hands of the three Fates. So too in days of scientific 

 illumination, we think of the dread three, but, clothing our 

 thoughts in other words, speak of life as determined by the 

 organism's legacy or inheritance, by force of habit or 

 function, and by the influences of external conditions or 

 environment. What the living organism is to begin with, 

 what it does or does not in the course of its life, and what 

 surrounding influences play upon it, — these are the three 

 Fates, the three Norns, the three Factors of Life. Organ- 

 ism, function, and environment are the sides of the bio- 



