PREFACE. 



THIS book is written for the sake of presenting simply and 

 concisely to students of biology and to general readers the 

 present-day standing of Darwinism in biological science, and 

 to outline for them the various auxiliary and alternative 

 theories of species-forming which have been proposed to aid 

 or to replace the selection theories. 



Our actual knowledge of the factors and mechanism of 

 organic evolution and our hypotheses and theories which 

 serve to fill in the present gaps in this knowledge have been 

 greatly added to and modified in the last few years. Much 

 that the general reader includes in his conception of organic 

 evolution, based on his reading of Darwin and Wallace and 

 Spencer, has been materially modified and some of it proved 

 untenable by modern investigation ; while much which had 

 no place in this earlier general understanding of evolutionary 

 method and process may now be confidently added to it. The 

 present time is one of unprecedented activity and fertility 

 both in the discovery of facts and in attempts to perceive 

 their significance in relation to the great problems of bio- 

 nomics. Both destructive criticism of old, and synthesis of 

 new hypotheses and theories, are being so energetically car- 

 ried forward that the scientific layman and educated reader, 

 if he stand but ever so little outside of the actual working 

 ranks of biology, is likely to lose his orientation as to the 

 trend of evolutionary advance. Precisely at the present mo- 

 ment is this modification of the general point of view and 

 attitude of philosophical biologists unusually important and 

 far-reaching in its relation to certain long-held general con- 

 ceptions of biology and evolution. This modification of the 

 general trend of evolutionary thought must also necessarily 



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