8io THE POPULAR SCIENCE MONTHLY. 



never been quite able to comprehend how this view, even if estab- 

 lished, militates against the transmissibility of acquired modifi- 

 cation; for, whatever theory of heredity we adopt, it shows us 

 rather the manner of the transmission, and therefore confirms its 

 possibility. But the fact of such transmissibility rests neither on 

 embryological nor theoretical grounds. It is a fact so fully dem- 

 onstrated in the history of our domestic animals and the history 

 of agriculture, that the skepticism of some of our great naturalists 

 and embryologists must be attributed to that ignorance of the 

 farmers' commonest experiences which is, unfortunately, a too 

 frequent attribute of tlie city-trained investigator, Darwin in 

 the beginning, and while the importance of natural selection was 

 growing in his mind, allowed little importance to use and disuse, 

 for the same reason that he subordinated external agencies ; viz., 

 that, in proportion as it acts on masses simultaneously, it must 

 diminish the importance of natural selection. Yet he allowed 

 more weight to it toward the end, and has furnished some of the 

 best evidence drawn from domestic animals of the transmission 

 of acquired characters affecting the dermal, muscular, osseous, 

 and nervous systems. Spencer has shown that inheritance of 

 functional modification is most easily observed and experiment- 

 ally proved in those parts which admit of easy observation and 

 comparison, as the dermal covering and the bones ; and that they 

 for the most part are beyond these tests in the muscular and 

 nervous systems. Yet he logically concludes : 



" Considering that unquestionably the modification of structure 

 by function is a vera causa, in so far as concerns the individual ; 

 and considering the number of facts which so competent an ob- 

 server as Mr. Darwin regarded as evidence that transmission of 

 such modifications takes place in particular cases ; the hypothesis 

 that such transmission takes place in conformity with a general 

 law, holding of all active structures, should, I think, be regarded 

 as at least a good working hypothesis." 



So far as entomology bears evidence, it confirms the fact that 

 modifications of structure due to use or disuse on the part of the 

 individual may be and are transmitted. These are easily ob- 

 served in the exo-skeleton, and, while the experimental proof is 

 yet limited, it is not wanting, especially in the history of apicult- 

 ure. Excessive use of any organ will develop or enlarge it at the 

 expense of other organs, just as disuse will cause a diminution or 

 atrophy thereof. The variation in the individual will be within 

 limits, but, when once the variation has set in, the tendency is 

 always to an increased variation in the same direction in the 

 descendants, especially if they continue the same use or disuse. 

 Here, again, however, it is difficult to separate the modification 

 due to individual effort, or want of effort, and the more general 



