486 THE GRAMMAR OF SCIENCE 



6' high, and this type would be permanently established, 

 even if selection ceased. Even with such a slowly repro- 

 ductive animal as man, two hundred years would be more 

 than enough for the change ; with birds, insects, and 

 many mammals six years might suffice for even greater 

 alterations in type. Our determination of the quantitative 

 strength of heredity is thus seen to give values quite 

 intense enough to produce rapid and permanent changes 

 of type, when selection is stringent. How stringent selec- 

 tion can be, even in the case of man, we shall see later. 



The reader must, of course, bear in mind that I have 

 here simplified down the problem to avoid extreme 

 length of analysis. I have supposed one organ or 

 character only selected, and that this selection does not, 

 by the principle of correlation, produce changes in other 

 organs unfavourable to the fitness or fertility of the 

 selected individuals. These points can also be allowed for 

 and dealt with by analysis, but my main object in this work 

 is only to show in the broadest outline how selection and 

 heredity combined lead to the establishment of new and 

 permanent types. Looked at from the social stand- 

 point, we see how exceptional families, by careful 

 marriages, can within even a few generations obtain 

 an exceptional stock, and how directly this suggests 

 assortative mating as a moral duty for the highly 

 endowed. On the other hand, the exceptionally de- 

 generate isolated in the slums of our modern cities can 

 easily produce permanent stock also ; a stock which no 

 change of environment will permanently elevate, and which 

 nothing but mixture with better blood will improve. But 

 this is an improvement of the bad by a social waste of 

 the better. We do not want to eliminate bad stock by 

 watering it with good,. but by placing it under conditions 

 where it is relatively or absolutely infertile. 



§ I 3. — On Exclusive Inheritance an'd the Law of Reversion 



So far we have been discussing a form of inheritance 

 in which each ancestor in the direct line contributes his 



