PREFACE. 7 



no definite tendencies so far as the corporeal forms 

 of organisms are concerned, but these tendencies have 

 all been developed — heredity amongst the rest — by the 

 environmental necessities of later time ; whilst varia- 

 tion or plasticity was a normal and necessary feature 

 of the original form of life, this constitutional elastic- 

 ity has been constantly bred out by the pressure of 

 circumstances, and the subsequent variation has come 

 to be more and more the result of definite environ- 

 ments. In some groups, in which the decline to- 

 wards extinction has now well progressed, or when 

 environments are very stable, organisms reproduce 

 themselves with considerable rigidity, so that it may 

 be said that like produces like. In some of the 

 variable groups, which, presumably, have not yet 

 reached the height of their development, it might 

 with equal truth be said that unlike produces unlike. 

 But in any event, the normal or original fact is con- 

 ceived to be that unlike produces unlike. At the 

 present time it would be truer to say that similar 

 produces similar. 



It has been doubted whether the conception of the 

 phyton be worth the while (Botanical Gazette, xxii., 

 501). The reviewer asks: "But do noteworthy dif- 

 ferences exist between the successive internodes of a 

 shoot?" It is by no means essential to the concep- 

 tion of the phyton that the different phytons upon 

 any branch shall be unlike ; although it should be 

 remembered that, as a matter of fact, no two branches 



