PREFACE 



The adaptation of animals and plants to the conditions 

 under which they live has always excited the interest, and 

 also the imagination, of philosophers and scientists ; for this 

 relation between the organism and its environment is one of 

 the most characteristic features of living things. The ques- 

 tion at once suggests itself : How has such a relation been 

 brought about ? Is it due to something inherent in the liv- 

 ing matter itself, or is it something that has been, as it were, 

 superimposed upon it ? An example may make my meaning 

 clearer. No one will suppose that there is anything inherent 

 in iron and other metals that would cause them to produce 

 an engine if left to themselves. The particular arrangement 

 of the pieces has been superimposed upon the metals, so that 

 they now fulfil a purpose, or use. Have the materials of 

 which organisms are composed been given a definite arrange- 

 ment, so that they fulfil the purpose of maintaining the 

 existence of the organism ; and if so, how has this been 

 accomplished ? It is the object of the following pages to 

 discuss this question in all its bearings, and to give, as far 

 as possible, an idea of the present state of biological thought 

 concerning the problem. I trust that the reader will not 

 be disappointed if he finds in the sequel that many of the 

 most fundamental questions in regard to adaptation are still 

 unsettled. 



In attempting to state the problem as clearly as possible, 

 I fear that it may appear that at times I have " taken sides," 



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