PREFACE ix 



and we have insisted on the value of seeking to correlate these 

 organismal advances with the changes in the world-environment. 

 Here again, as also in XII, there is emphasis on the idea of the 

 Cell-Cycle. Perhaps we may also refer to a fresh note struck in 

 our brief discussion of the old questions of the possibility of life 

 beyond our terrestrial limits. 



In IX (Factors in Evolution) we have made not a few sugges- 

 tions: — such as (a) the interpretation of evolutionary Variations, 

 from small to greater, in terms of alternatives in the ratio between 

 anabolism and katabolism; (b) the exposition of physiological 

 trends of Variation in Plants, with particular reference to the 

 relation between nutrition and reproduction (a conception also 

 applicable as regards the evolution of animals) ; (c) the definition and 

 illustration of the role of "Nurture" in individual development, 

 and in racial evolution ; (d) the analysis of reversionary phenomena, 

 carried further than usual; {e) the further working out of the 

 Darwinian idea of the Web of Life as of selective value; and (/) the 

 clearing up of ideas and terminology throughout the main sciences, 

 in the section on "Different Kinds of Evolution". 



We need not continue our instances of more or less personal 

 contributions in the later chapters of the book (X-XIII) and its 

 three appendices, though it is perhaps here that they are most 

 numerous, and sometimes most adventurous; since even to intrusion 

 upon the current discussion of the Relativity-doctrine; and to the 

 suggested projection of the technique of our treatment of Life 

 towards the presentment of main ideas of the physical sciences. 

 This indeed some of their recent thinkers, as notably Whitehead, 

 are already beginning to do. 



A final personal word. While the writers have been collabo- 

 rating with substantial agreement since (and even before) their 

 Evolution of Sex (1889), their work and thought have mostly been 

 in very different and distinct fields; whence naturally much dis- 

 cussion as to the scope, perspective, and co-adjustment of their 

 respective studies of Life, and in its Evolution. Thus while every 

 chapter, section, and even paragraph has been mutually criticised, and 

 sofar adjusted, certain shades of difference remain, as especially of 

 outlooks, respectively more of Neo-Darwinian and Neo-Lamarckian 

 tendencies. Still, our book has also gained something towards its 

 treatment of problems still far from complete solution, from ex- 

 periences in Nature so largely different yet mainly complemental ; 

 and from lives so long and often separate, yet with renewing co- 

 operations in comradeship. 



We must thank our old artist-friend, Mr. William Smith, for his 

 careful skill in making the illustrations, many from actual speci- 

 mens; and also various authors and publishers for certain figures. 

 Thus, too, Messrs. Thornton Butterworth, Ltd., for re-drawing of 

 our End-Papers from diagrams in our Biology in their Home 

 University Library. Finally, we thank our publishers for their 

 considerate patience with the authors' delays over their long and 

 ambitious endeavour. T ^ T 



p/g. 



