THE FACTORS OF ORGANIC EVOLUTION. jj-j 



its reaction upon other parts entails changes in them. This reaction 

 is effected through the changes of function involved. If the changes 

 of structure produced by such changes of function, are inheritable, 

 then the re-adjiistraent of parts throughout the organism, taking place 

 generation after generation, maintains an approximate balance ; but if 

 not, then generation after generation the organism must get more and 

 more out of gear, and tend to become unworkable. 



Further, as it is proved that change in the balance of functions 

 registers its effects on the reproductive elements, we have to choose 

 between the alternatives that the registered effects are irrelevant to the 

 particular modifications which the organism has undergone, or that 

 they are such as tend to produce repetitions of these modifications. 

 The last of these alternatives makes the facts comprehensible ; but the 

 first of them not only leaves us with several unsolved problems, but is 

 incongruous with the general truth that by reproduction, ancestral 

 traits, down to minute details, are transmitted. 



Though, in the absence of pecuniary interests and the interests in 

 hobbies, no such special experiments as those which have established 

 the inheritance or fortuitous variations have been made to ascertain 

 whether functionally-produced modifications are inherited ; yet certain 

 apparent instances of such inheritance have forced themselves on 

 observation without being sought for. In addition to other indications 

 of a less conspicuous kind, is the one I have given above — the fact 

 that the apparatus for tearing and mastication has decreased with de- 

 crease of its function, alike in civilized man and in some varieties of 

 dogs which lead protected and pampered lives. Of the numerous 

 cases named by Mr. Darwin, it is observable that they are yielded not 

 by one class of parts only, but by most if not all classes — by the der- 

 mal system, the muscular system, the osseous system, the nervous sys- 

 tem, the viscera ; and that among parts liable to be functionally modi- 

 fied, the most numerous observed cases of inheritance are furnished by 

 those which admit of preservation and easy comparison — the bones : 

 these cases, moreover, being specially significant as showing how, in 

 sundry unallied species, parallel changes of structure have occurred 

 along with parallel changes of habit. 



What, then, shall we say of the general implication ? Are we to 

 stop short with the admission that inheritance of functionally-produced 

 modifications takes place only in cases in which there is evidence of 

 it ? May we properly assume that these many instances of changes 

 of structure caused by changes of function, occurring in various tis- 

 sues and various organs, are merely special and exceptional instances 

 having no general significance ? Shall we suppose that though the 

 evidence which already exists has come to light without aid from a 

 body of inquirers, there would be no great increase were due attention 

 devoted to the collection of evidence ? This is, I think, not a reasona- 

 ble supposition. To me the ensemhle of the facts suggests the belief, 



