INTEA-SELECTION OR SELECTION AMONG TISSUES 249 



case the particular species would be victorious which was best adapted 

 to the local conditions. But each would thrive best in the region in 

 which it was superior to the others, and very soon the three species 

 would be distributed as they were in the land from which they 

 came — in the plains, the high lands, and the mountain forests. This 

 would be the result of a struggle between the three species, not 

 between individuals ivithin each species, and it could not therefore 

 bring about an improvement of a single species, but only the local 

 prevalence of one or another. The characters which made one 

 species adapted for the plain, another for the mountain forest were 

 already there; they can only be referred to personal selection, which 

 brought about the adaptation of their ancestors in the course of ages 

 to the conditions of their life. Something similar is true of the 

 adaptations of the tissues ; the differentiation of the individual kinds 

 of cells is an ancient inheritance, and depends upon personal selection, 

 but their distribution and arrangement into specially adapted tissues, 

 so far as there is any plasticity at all, depends upon histonal selection. 

 Obviously, however, only as far as the tissue is plastic, that is, with 

 the power of adjusting itself to particular local conditions. Only 

 adaptations of this kind can be referred to histonal selection; the 

 ground-plan, even of the most complicated tissue, such as the large 

 glands of mammals, the kidneys, the liver, and so on, must have been 

 implicit in the germ, and must therefore be referred to personal 

 selection. A precise limitation of the respective spheres of action 

 of personal selection and histonal selection is not possible as yet, since 

 hardly any investigations on the subject are available. 



Koux undoubtedly over-estimated the influence of his ' struggle 

 of parts ' when he believed that the most delicate adaptations of the 

 different kinds of cells depended on it. I admit that, for a con- 

 siderable time, I made the same mistake, until it became clear to me, 

 as it did first in regard to the sex-cells, that this is not, and cannot be 

 the case. How, for instance, could the diverse and minutely detailed 

 adaptations of the sex-cells — which we are to discuss in a subsequent 

 lecture — have arisen in this way 1 As far as the individual sperm-cell 

 is concerned, it is a matter of indifference whether its head is a little 

 thinner or thicker, its point a little sharper or blunter, its' tail a little 

 stronger or weaker. This does not decide whether the cell is to 

 thrive better, or to occur in greater numbers than some other variety. 

 But it does decide whether it is to be able to penetrate through the 

 minute micropyle, or through the firm egg- envelope, into the egg, 

 there to eflFect fertilization. An individual with less well formed 

 sperm-cells will be able to fertilize fewer eggs, and therefore to leave 



