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,” 
vii 
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