6i8 PRACTICAL PROBLEMS 



Darwin's experiments. 1 Fortunately so far as plants are con- 

 cerned we are not without some accurate data tending to show 

 the actual effect of inbreeding upon the two most important 

 characters here under discussion, namely, vigor and fertility, 

 and for a great variety of species. The experiments are too 

 extensive to fully discuss even by abstract, covering as they do 

 some fifty-seven species, belonging to fifty-two genera ; 2 but 

 their results may be briefly stated. 



The careful study of these experiments shows the following 

 facts : (i) that in general, and without a doubt, crossed forms 

 (both they and their offspring) are, on the average, much more 

 fertile and far more vigorous than are the self-fertilized ; (2) but 

 that this is not true of all species, nor is it true of all individuals, 

 even within those species most sensitive to inbreeding. 



Thus, of the 83 species tested for height, 26, or nearly one 

 third, were either within 5 per cent of the height of their cross- 

 bred companions, or else exceeded them in height. Of these 26 

 cases, however, he concludes that 14 were actually inferior, if 

 not in height, at least in other respects, leaving 12, or one 

 seventh of all, that quite clearly were not inferior when inbred, 

 and in some cases were decidedly the better for it. 3 Concluding, 

 Darwin remarks : 



Therefore if we exclude the species which are approximately equal, there 

 are thirty-seven species in which the mean of the mean heights of the 



1 Charles Darwin, Cross and Self Fertilization in the Vegetable Kingdom, 

 p. 482 [D. Appleton & Company]. It is unfortunate that we do not possess 

 equally full and exact data as to inbreeding among animals, but at this point our 

 knowledge is limited to general results and to individual experiences. The marked 

 success of close breeding and even inbreeding in our herds is attributed to the 

 special skill of a " master breeder." That this is not the full explanation is shown 

 by the experience of the United States Bureau of Animal Industry. Some years 

 ago it became necessary to remove the stock of guinea pigs to new quarters a 

 considerable distance away. A severe storm was encountered en route and only a 

 few pigs were saved. From these few, and with no infusion of outside blood, the 

 present stock is descended, and the writer is credibly informed that the stock is 

 exceptionally vigorous and fertile. 



2 The student desiring the data upon the effects of cross- or self-fertilization in 

 general should read chap, vii, pp. 238-284, of Darwin's Cross and Self Fertiliza- 

 tion, etc. ; for data concerning the effect upon seed production he should read 

 chap, ix, pp. 312-355 ; and for data concerning other effects, chap, viii, pp. 285-31 1 ; 

 for detailed reports of different species see chaps, ii-vi, especially ii. 



3 Darwin, Cross and Self Fertilization, etc., pp. 279-283, 



