BASIS OF THE PROBLEM 57 



The importance of this distinction lies in the fact that the only 

 method of obtaining the highly complex substances necessary 

 for animals is to feed upon the tissues of other animals or plants. 

 It is obvious that every animal species cannot feed upon some 

 other animal species ; in the end animals as a whole must depend 

 upon plants because plants alone are able to elaborate the sub- 

 stances which animals need. In a sense, therefore, animals are 

 parasitic upon plants ; in any case the existence of animals is 

 bound up Avith the continued existence of plants. This inter- 

 dependence of living organisms runs all through the conditions 

 under which species in a state of nature live and takes a variety 

 of shapes. The dependence of one organism upon another is 

 largely connected with the question of the provision of appropriate 

 surroundings which are often only found in the proximity of 

 certain other species. Many species can only flourish in the 

 neighbourhood of trees. The interdependence between certain 

 species is very intimate. There are many examples of what is 

 known as symbiosis, as when a certain species of sea anemone 

 hves on the back of a particular species of crab. Again, parasites, 

 which alternate between one host and another, are dependent 

 upon finding a member of a particular species at a certain time 

 in their life-history, as otherwise they perish. Many examples of 

 interdependence are within common knowledge, and, bearing 

 this feature of organic life in mind, we may go on to ask how it is 

 that, through the elimination of a great proportion of the young 

 of every species, the number of adults remains upon the whole 

 constant. 



14. Taking animals first, it is probable that the most common 

 cause of elimination lies in the fact that the young of all species 

 are consumed by members of other species. It is difficult to 

 estimate even roughly the relative importance of the various 

 causes of elimination ; ' the causes ', says Darwin, ' which check 

 the natural tendency of each^pecies to increase are most obscure '.^ 

 The particular factor mentioned, however, certainly takes a very 

 prominent place. The young of marine and fresh-water animals 

 almost all form food for other species and are obviously exposed 

 to attack. So too, though perhaps not to so great a degree, are 

 the young of terrestrial animals ; whether we think of the larvae 

 of insects or the eggs of birds, we find that they are in most cases 



* Darwin, Origin of Species, p. 49. 



