Page:George Archdall Reid 1896 The present evolution of man.djvu/127

Rh mony with a complex and heterogeneous environment, but that still further to increase this harmony with changeful surroundings, it has developed in the structures an immense power of varying in the individual under appropriate stimulation, the amount of the power of varying as regards each structure being in proportion to the importance of its presence there as a factor in survival; so that some structures, such as teeth, have little or no power of varying under stimulation, since their function is such that in relation to it week by week and year by year the environment varies little; whereas other structures, such as skin or muscle, have greater powers of varying, since their functions are such that in relation to them week by week or year by year the environment varies more. By this power of varying along certain lines under appropriate stimulation, all the multitudinous structures of the high animal body are brought into exquisite harmony, each one with all the others; and therefore, should an animal vary in such a manner as to have a structure—say a certain muscle—larger than the corresponding structure in its progenitors, or rather, to speak more precisely, should an animal vary in such a manner that the structure develops under a "normal" amount of stimulation more than with the same amount of stimulation it developed in the progenitors, then, since this extra development puts an extra strain on all co-operative structures, there results a proportionate increase of size in them all. On the other hand, should an animal vary in such a manner that the structure develops under a normal amount of stimulation less than with the same amount of stimulation the corresponding structure developed in the progenitors, then, since the lesser development puts a smaller strain on all cooperative structures, there results a proportionate and