298 PHYSIOLOGICAL DEVELOPMENT. 



species, which no differentiation has a direct tendency to 

 initiate. Hence we must regard the total results as due to a 

 plexus of influences acting simultaneously on the individual 

 and on the species : some chiefly affecting the one and some 

 chiefly affecting the other. 



[Note. — In Nature for June 11, 1896, Dr. Maxwell Mas- 

 ters, in an essay on " Plant Breeding," names an instructive 

 fact concerning the production of varieties by selection of 

 slightly divergent forms. He says : — 



" To the untrained eye, the primordial differences noted are 

 often very slight; even the botanist, unless his attention be 

 specially directed to the matter, fails to see minute differences 

 which are perceptible enough to the raiser or his workmen. 

 Nor must it be thought that these variations, difficult as they 

 are to recognise in the beginning, are unimportant. On the 

 contrary, they are interesting, physiologically, as the potential 

 origin of new species, and very often they are commercially 

 valuable also. These apparently trifling morphological dif- 

 ferences are often associated with physiological variations 

 which render some varieties, say of wheat, much better 

 enabled to resist mildew and disease generally than others. 

 Some, again, prove to be better adapted for certain soils or 

 for some climates than others ; some are less liable to injury 

 from predatory birds than others, and so on." 



Thus we are shown that, to a much greater degree than 

 might be supposed, minute changes of forms and functions 

 in one part of a plant are correlated with changes of forms 

 and functions throughout it. The inter-dependence — that is 

 to say, the physiological integration — is very close at the 

 same time that it is very complex. 



Here while naming these facts in illustration of physio- 

 logical integration in plants I name them because they 

 illustrate an important truth bearing upon the general ques- 

 tion of heredity which I have dealt with in Appendix G-, and 

 to which I now especially draw attention.] 



