380 EVOLUTION OP PHYSICAL CHAEACTEES 



of the world in so obvious a manner that it need not be further 

 discussed. 1 



Warfare following upon migration may exterminate whole 

 races. The Bushmen were apparently on the verge of extinction 

 at the hands of the invading Bantu peoples. In this manner 

 a peculiar physical type may be lost and another may become 

 of relatively greater prevalence. Migration may also be followed 

 by any degree of racial mixture. Many rash statements have been 

 made by historians as to the biological results of crossing. The 

 consequences actually observed to follow upon a mixture of races 

 are to so large a degree the result of a conflict of traditions — a 

 subject with which we shall deal in a later chapter — that the 

 purely biological effects of crossing may be very largely obscured. 

 Bearing this in mind we may briefly refer to the conclusions 

 to be derived from recent biological work which bear on the 

 problem. Eoughly speaking there are two possible kinds of 

 crosses between races. Pirst there are crosses between the most 

 clearly distinguished varieties such as white and black. Heterosis, 

 or hybrid vigour, will be exhibited in a marked fashion in the 

 first generation. Heterosis, the underlying cause of which has 

 only recently become apparent, is always at its height in the first 

 cross. The increase of vigour, however, is not long maintained in 

 subsequent generations. Further, each type, such as those which 

 we are considering, has a series of character complexes, built 

 up through ages of selection and compatible with one another, 

 and by crossing such complexes are broken apart. The chance 

 of gain, on the other hand, through the favourable re-combination 

 of characters is small. On the average, therefore, the result of 

 such a cross is unfavourable. There may also be crosses between 

 races exhibiting less differences. Again, heterosis will be visible 

 on crossing. But in distinction to the results of the former kind 

 of cross, the other results may not be unfavourable. Great 

 variabihty may follow such a cross and this is on the whole 

 advantageous. Valuable character re-combinations may also 

 come to light. Thus we may say that, so long as there is not too 

 great a difference between the races which cross, the results are 



' There has been much discussion as to the effect upon the population of France 

 owing to the Napoleonic wars. The facts have been summed up by Havelock 

 Ellis {Essays in War Time, pp. 33 ff.), and it appears that without any question the 

 average physical constitution of the French was lowered ; most kinds of infirmities, 

 for instance, became more frequent. 



