i844- l8 S 8 ] DISUSE 87 



tion would allow the existence of only a few species in such Letter 43 

 genera. Whenever we meet (which will be on the 23rd [at 

 the] Club) I shall much like to hear whether this strikes you as 

 sound. I feel all the time on the borders of a circle of truism. 

 Of course I could not think of such a request, but you might 

 possibly : — if Bentham does not think the whole subject 

 rubbish, ask him some time to pick out the dozen most 

 anomalous genera in the Leguminosae, or any great order of 

 which there is a monograph by which I could calculate the 

 ordinary percentage of species to genera. I am the more 

 anxious, as the more I enquire, the fewer are the cases in 

 which it can be done. It cannot be done in birds, or, I fear, 

 in mammifers. I doubt much whether in any other class of 

 insects [other than Curculionidae]. 



I saw your nice notice of poor Forbes in the Gardeners' 

 Chronicle, and I see in the Athenceum a notice of meeting on 

 last Saturday of his friends. Of course I shall wish to subscribe 

 as soon as possible to any memorial. . . . 



I have just been testing practically what disuse does in 

 reducing parts. I have made [skeletons] of wild and tame 

 duck (oh the smell of well-boiled, high duck !), and I find the 

 tame duck ought, according to scale of wild prototype, to 

 have its two wings 360 grains in weight ; but it has only 317, 

 or 43 grains too little, or A of [its] own two wings too little 

 in weight. This seems rather interesting to me. 1 



P.S. — I do not know whether you will think this worth 

 reading over. I have worked it out since writing my letter, 

 and tabulate the whole. 



21 orders with 1 genus, having 7*95 species (or 4'6 ?). 



29 orders with 2 genera, having I5'o5 species on an average. 



23 orders each with 3 genera, and these genera include on an average 



8'2 species. 

 20 orders each with 4 genera, and these genera include on an average 



i2"2 species. 

 27 orders each with above 50 genera (altogether 4716 genera), and 



these genera on an average have 997 species. 



1 On the conclusions drawn from these researches, see Mr. Piatt Ball, 

 The Effects of Use and Disuse (Nature Series), 1890, p. 55. With regard 

 to his pigeons, Darwin wrote, in Nov. 1855 : " I love them to that extent 

 that I cannot bear to kill and skeletonise them." 



