1800 ] «J^ [Bailey. 



the very nature of its constitution, could not exactly reproduce itself. 

 The burden of proof has been thrown upon those who attempt to explain 

 the initial origin of differences, but it should really be thrown upon those 

 who assume that life-matter was originally so constructed as to rigidly 

 recast itself iuto one mould in each succeeding generation. I see less 

 reason for dogmatically assuming that like produces like than I do for 

 supposing that unlike produces unlike. 



I advanced this proposition a year ago in my Plant- breeding (pp, 9, 

 10), and I am now glad to find, since writing the above paragraph, 

 that H. S. Williams has reached similar conclusions in his new Geological 

 Biology. Pie regards mutability as the fundamental law of organisms, 

 and speaks of the prevalent notion that organisms must necessarily repro- 

 duce themselves exactly as "one of the chief inconsistencies in the preva- 

 lent conception of the nature of organisms." "While the doctrine of 

 mutability of species has generally taken the place of immutability," he 

 writes, "the proposition that like produces like in organic generation is 

 still generally, and I suppose almost universal!}', accepted. It therefore 

 becomes necessary to suppose that variation is exceptional, and thatsome 

 reason (or the accumulation of variation is necessary to account for the 



great divergencies seen in different species The search has been 



for some cause of the variation ; it is more probable that mutability is the 

 normal law of organic action, and that permanency is the acquired law." 

 I do not suppose that Professor Williams makes definite variation an inhe- 

 rent or necessary quality of organic matter, but that this matter had no 

 original herediiary power and that its form and other attributes in suc- 

 ceeding generations have been moulded into the environment, and that 

 the burden of proof is thrown upon those who assume that life-matter 

 was endowed with the property that like necessarily produces like. At 

 till events, this last is my own conception of the modification of the 

 streams of ascent. 



In other words, I look upon heredity as an acquired character, the same 

 as form or color or sensation is, and not as an original endowment of 

 mailer. The hereditary power did not originate until for some reason it 

 was necessary for a given character to reproduce itself, and the longer any 

 form or character was perpetuated, the stronger became the hereditary 

 power. 



It is now pertinent to inquire what determined the particular diflferen- 

 ces which we know to have persisted. The mere statement that some 

 forms became sessile or attached to the earth, and that others became 

 or remained motile, is an assumption that these differences were direct 

 adaptations to environment. Every little change in environment incited 

 a corresponding change in the plastic organization ; and the greater and 

 more various the changes in the physical attributes of the earth with the 

 lapse of time, the greater became the modifications in organisms. I be- 

 lieve, therefore, that the greater part of present diflerences in organisms 

 are the result directly and indirectly of external stimuli, until we come 



