HIGH SCHOOL ZOOLOGY. 285 



21. The theory stated in the preceding paragraphs is that of 

 the Origin of Species by Natural Selection, associated with the 

 names of Darwin and Wallace ; it will be observed that while 

 resting iipon the large amount of variation offered, it does not at- 

 tempt to explain the cause of such variation. This is attributed 

 by certain American zoologists, — of whom Cope is the chief rep- 

 resentative — to the direct action of the environment, for ex- 

 ample, the gradual preponderance assumed by the central digits 

 in the Ungulates would be explained by the greater strain 

 received by those reaching the ground. Strict Darwinists do 

 not consider such an explanation to be sufficient, l)eca>ise there 

 are many instances of protective resemblance and mimicry 

 where just as remarkable modifications of form are to be met 

 with, which could not be attributed to such a direct action of 

 the environment. On the other hand, we have met in the pre- 

 ceding chaj)ters with so many instances of the adai)tation of the 

 organism to its habits {vide Index, adaptation) that it seems 

 difficult to believe that such remarkable corres|)ondence should 

 only be the result of selection from variations tending to occur 

 In every direction. 



22. Not only have we to contemplate the effect of comi)etition 

 between different individuals as favounible to change in organic 

 nature, but also the competition of different species This is 

 readily seen in a garden which, if left to itself, soon becomes 

 ovei'grown with weeds — those forms of plants which have either 

 better means of protecting or dispersing themselves, or are aVjle 

 to cope with less favourable conditions of life than the plants for- 

 merly cultivated. It is easy to see, that such competition, where 

 species come into contact for the first time, may lead to extinc- 

 tion of the form less able to protect itself ; indeed the extinc- 

 tion of past forms of life is to be explained partly by such com- 

 petition, and partly by reason of some forms being specialised 

 to such a narrow range of conditions, that they have become 



