J. p. LOTSY. 
,,wikl duck and australian dog alwajs becoraing^ wlien bred for oiie or 
„ei few générations in confinement, mottled in their colours, wlien we 
,,see people living in certain districts or circumstances becoming subject 
„to an hereditary taint, to certain organic diseases and consumption 
„(on ne savait pas encore à cette époque que des bactéries pouvaient 
^provoquer des maladies) or plica polonica, we naturally attribute such 
^changes to the direct efFect of known or unknown agencies actiug for 
,,one or more générations on the parents. It is possible that a multitude of 
„peculiarities may thus bedirectly caused by unknown external agencies/' 
Darwin attachait beaucoup plus d'importance à Tinfluence indirecte 
des conditions extérieures : 
„1 may add judging from the vast number of new varieties of plants 
„which have been produced in the same districts and under nearly the 
„same routine of culture^ that probably the indirect effects of domesti- 
,,cation in making the organisation plastic is a much more efficient 
,,source of variation than any direct effect which external causes may 
„have on the colour, texture or form of each part/^ 
Néanmoins, la véritable cause de la variabilité est restée obscure 
pour Darwin jusque dans sa vieillesse. 
C'est ainsi que le 19 juillet 1881 ') il écrivit ^) à Semper: 
„and it is really surprising how little effect he (Hoffmann) produced 
„by cultivating certain plants under unnatural conditions, as the présence 
„of sait, lime, zinc etc. etc. during générations. Plants moreover 
„were selected which were the most likely to vary under such conditions 
„judging from the existence of closely-allied forms adapted to thèse 
„conditions. No doubt I originally contributed to little w^eight to the 
„direct action of conditions, but Hoffmann's paper has staggered me. 
„Perhaps hundreds of générations of exposure are necessary. It is a 
„most perplexing subject. I wish I was not so old and had morestrength, 
„for I see lines of research to follow I still must believe that 
„changed conditions give the impulse to variability, but that they act 
„in most cases in a very indirect manner." 
Si Von admet que cet agent indirect est le croisement^ qui est, comme 
on sait, cousidérahlement favorisé par la culture, toutes les difficultés 
disparaisseAit , même celle de la remarqtce de Darwin, qu'il faut parfois 
') Dauwin mourut le 19 avril 188.2. 
Life and Letters, vol. III, p. 345. 
