140 COMPARATIVE PHYSIOLOGY. 



given environment survives. But Darwin has probably fixed 

 his attention too closely on this principle and attempted to ex- 

 plain too much by it, as well as faUed to see that there are 

 other deeper, facts underlying it. Variation, which this author 

 scarcely attempted to explain, seems to us to be the natural re- 

 sult of the very conditions under which living things have an 

 existence. Stable equilibrium is an idea incompatible with our 

 fundamental conceptions of life. Altered function implies al- 

 tered molecular action, which sometimes leads to appreciable 

 structural change. From our conceptions of the nature of liv- 

 ing matter, it naturally follows that variation should be great- 

 est, as has been observed, under the greatest alteration in the 

 surroundings. 



We are but very imperfectly acquainted as yet with the 

 conditions under which life existed in the earlier epochs of the 

 earth's history. Of late, deep-sea soundings and arctic explo- 

 rations have brought surprising facts to light, showing that 

 living matter can exist under a greater variety of conditions 

 than was previously supposed. Thus it turns out that light is 

 not an essential for life everywhere. We think these recent 

 revelations of unexpected facts should make us cautious in 

 assuming that life always manifested itself under conditions 

 closely similar to those we know. Variation may at one period 

 have been more sudden and marked than Darwin supposes; 

 and there does seem to be room for such a conception as the 

 "extraordinary births" of Mivart implies; though we would 

 not have it understood that we think Darwin's view of slow 

 modification inadequate to produce a new species, we simply 

 venture to think that he was not justified in iasisting so strongly 

 that this was the only method of Nature ; or, to put it more 

 justly for the great author of the Origin of Species, with the 

 facts that have accumulated since his time he would scarcely 

 be warranted in maintaining so rigidly his conviction that 

 new forms arose almost exclusively by the slow process he has 

 so ably described. 



We must allow a great deal to use and effort, doubtless, and 

 they explain the origin of variations up to a certain point, but 

 the solution is only partial. Variations must arise as we have 

 attempted to explain, and use and disuse are only two of the 

 factors amid many. Correlated growth, or the changes in one 

 part induced by changes in another, is a principle which, 

 though recognized by Darwin, Cope, and others, has not, we 



