324 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



state, but which had been hidden, thus become manifest. The inbreeding as 

 such is not necessarily injurious, provided the genetic constitution of both 

 parents is a very favorable one. 



The deterioration caused by inbreeding in animals can, perhaps, to a certain 

 extent be mitigated and delayed through continuous selection of the most 

 vigorous individuals of the inbred strain for breeding purposes ; such a selec- 

 tion was made in the breeding experiments in rats by H. D. King and it is 

 possible that by these means a severe deterioration was avoided, at least for 

 a long time. We may assume that presumably in using the strongest in- 

 dividuals, in most cases also the most allozygous (or heterozygous, according 

 to the usual terminology) individuals were chosen, and thus the approach 

 to an autozygous condition was delayed. Evidently a homoiogenous com- 

 bination of genes in a zygote and in the individual subsequently formed is 

 most favorable for the best development of a higher animal organism ; con- 

 versely, a condition of autosis in fertilization may lead to deterioration. In 

 this respect gene combinations in the fertilized ovum differ from the com- 

 binations of gene derivatives, the individuality differentials, as they are accom- 

 plished in transplantation of tissues. Here, as we have seen, the autogenous 

 combination is the most favorable one; a syngenesio-, and still more so, 

 a homoio- and a hetero-combination are injurious. However, it is possible 

 also that a combination of genes derived from two unlike parents may lead 

 to a summation of two beneficial conditions and that this summation may 

 produce favorable physiological conditions in the hybrid. Such an effect 

 seems to have been observed by Robbins in two races of tomatoes in which 

 the hybrid F x presumably possessed the combined ability of both parents to 

 synthesize certain vitamines B. 



From a physiological point of view, it has also been suggested that a 

 combination of genes which differ within a certain range of intensity in the 

 Fj generation, leads to the development in the embryo of a substance or of 

 substances, which are slightly different from those to which the fertilized 

 ovum and the developing embryo are adapted, and that this condition if 

 present within a certain range of concentration exerts a stimulating effect, 

 while a substance which exceeds a certain degree of strangeness may cause 

 injurious effects. This formulation recalls the so-called Arndt-Schultz rule, 

 according to which very small doses of toxic substances, instead of having 

 an injurious effect, on the contrary, may exert a stimulating effect. It was 

 especially Lohner who, in comparing the effects of inbreeding and of fer- 

 tilization of ova by less nearly related sperm, applied the Arndt-Schultz rule 

 to their analysis. We should then attribute the advantage of homoio-fertiliza- 

 tion over close inbreeding to the stimulation caused by a greater mutual 

 strangeness of the genes in the former process, as compared with the great 

 similarity of the genes in the latter. Furthermore, cross-fertilization between 

 different subspecies, or between certain very nearly related species, on ac- 

 count of the still greater mutual dissimilarities of the combining genes, might 

 be even more beneficial and exert also the effects characteristic of heterosis 

 (allosis) ; however, if the dissimilarities between the genes exceed a certain 



