134 DARWINISM AND THE PROBLEMS OF LIFE 



complicated and higher species when they reach special 

 localities. We shall see this in a later chapter. 



The physical conditions of our planet are always 

 changing, it is true, and on this account the animals of 

 to-day are, generally speaking, different from those of 

 former times. But, on the one hand, there are un- 

 doubtedly spots on the earth in which the changes have 

 not been very great, such as certain parts of the deep 

 sea ; and, on the other hand, many animals can retain 

 their simple form in spite of considerable changes. 

 Thus we can understand why the very simple organisms 

 from which all living things have been developed are 

 still found in every drop of water. 



We will inquire further into the conditions that bring 

 about the conversion of part of a species into a more 

 complicated one, and leave the other part at the same 

 stage of organisation. We have to- day not only the 

 final twigs of the tree of evolution. We have worms, 

 insects, vertebrates — in a word, living things at every 

 stage of organisation. But most animals have changed, 

 even when they have not gone beyond the range of this 

 fundamental form. The reptiles that live to-day differ 

 from those of former times, yet they are reptiles, and 

 only a part of them chanced to get into such special 

 conditions that birds were developed from them ; and 

 these were so favourably placed that they grew in 

 number and variety of species, as the new element they 

 had found had room for adaptation in the most diverse 

 directions. But geology tells us of animals that have 

 persisted in almost the same form for incalculable 

 periods ; and we must assume that some very exceptional 



