EVOLUTION OF PHYSICAL CHAKACTEES 377 



We may glance at the chief features of these changes. In the 

 last section we mentioned various factors infanticide of the 

 deformed, customs with regard to the treatment of children, 

 the neglect and exposure of children, the general conditions of 

 life, and so on all of which have as their result the cutting-off 

 not merely of the deformed and the monstrosities but also of all 

 departures from the type best fitted to contend with the climatic 

 environment and the conditions of daily life. Some of these 

 factors such as infanticide disappear ; the action of others 

 becomes, if anything, reversed ; not only do the less well -fit ted 

 types have a better chance of survival than before, because the 

 conditions are less rigorous, but they suffer little or no dis- 

 advantage owing to the fact that the conditions have been 

 artificially rendered almost as favourable for the less-fitted as 

 for the better-fitted. To take two examples, not only are men 

 with defective eyesight not eliminated, but they are by the 

 invention of spectacles placed in as good a position as those with 

 perfect eyesight. Similarly a woman with a narrow pelvis is, 

 owing to the advance in surgical skill, enabled to bear children 

 and to transmit her peculiarity to her daughters. 



It should not be forgotten that, in spite of the gradual lessening 

 of the rigour of selection, selection owing to climate and the general 

 conditions of life still continues. But another factor has within 

 this period come to assume a preponderating importance, and 

 that is selection through disease. It was pointed out that diseases 

 may be roughly distinguished into those due to the attacks of 

 parasites and those due to structural defects. It is to diseases of 

 the first kind that the greater part of selection, which occurs in 

 the third period, is due. Some figures were given in an earlier 

 chapter showing how large a proportion of deaths at the present 

 day is due to one or other of these diseases. In the last chapter 

 it was shown that men differ in their susceptibility to these diseases, 

 and selection has thus very largely come to take the form of the 

 elimination of the more susceptible, and of the favouring of the 

 naturally immune and of those who have a power of resisting 

 disease and of acquiring immunity. There has therefore come to 

 be an increasingly heavy premium upon the type of constitution 

 which can resist disease, and a strong constitution in this sense is 

 not necessarily the same as a strong constitution among primitive 

 races where the premium is rather upon muscular strength, 



