CH. XIX Influence of Habits and Surroundings 317 



their hosts, and some internal parasites produce slight 

 modifications of structure. Interesting also are the shelters 

 or domatia of some plants, within which insects and mites 

 find homes. 



We can speak more confidently about the influence of 

 surroundings than we could in regard to the influence of 

 use and disuse, because the ascertained facts are more 

 numerous. Those interested in the theoretical importance 

 of these facts should attend to the following considerations. 



It is essential to distinguish between cases in which 

 we know that external conditions influence the organism 

 and those in which we think they may have done so. Thus 

 it is probable that the degeneracy and other peculiarities of 

 many parasites are results of external influence and of 

 feeding, and also in part of disuse, but we cannot state 

 this as a fact. 



Most of the observations on the influence of external 

 conditions give us no information as to the transmissi- 

 bility of the results. It is not enough to know that a 

 peculiarity observed to occur in peculiar surroundings was 

 observed to recur in successive generations living in the 

 same surroundings. For (i) it might be an indefinite 

 variation — a sport due to some germinal peculiarity — 

 which happened to suit. In such a case it would be 

 transmissible, but it would not be a change due to the 

 environment. And (2) even when it has been proved that 

 the peculiarity is due to the direct influence of the environ- 

 ment, and observed to recur in successive generations, still 

 its transmissibility is not proven, for it may be hammered 

 on each successive generation as it was on the first. We 

 can say little about the transmissibility or evolutionary 

 importance of changes of structure due to surroundings 

 because most of the observations were made before the 

 scepticism as to the inheritance of acquired characters 

 became dominant. Only in a few cases, such as that of the 

 brine-shrimps, was the cumulative influence traced through 

 many generations. In dearth of facts we should not be 

 confident, but eager for experiment. 



Surroundings may influence the organism in varying 



