A Suggestion on Extinction. 465 



close in-breeding diminishes vigour and fertility," and he* 

 himself made many experiments tending to corroborate this 

 opinion. I have also found from other sources that this is a 

 very general opinion amongst farmers and breeders of poultry 

 and cattle. The constant attempt on the part of nature to 

 ensure cross-fertilisation amongst fixed animals and plants 

 is sufficiently convincing that there must be a deep-seated 

 factor underlying in-breeding which tends to its elimination 

 as far as is possible. The effect of close in-breeding in man 

 himself appears to affect the very system iu which he is most 

 specialised. That this is the general opinion is evident from 

 the laws of all civilised nations. Some medical men, how- 

 ever, are inclined to think that there is no bad result unless 

 there is a neurotic tendency already present in the family. 

 This would only lead one to ask what produced the neurotic 

 tendency. It can only have occurred as a variation, and 

 may not the same cause which produces the neurotic tend- 

 ency be also the cause which produces the ill-effects due to 

 too close in-breeding ? I have myself, in several cases of 

 families in which all or several of the children were deficient 

 in some part of the nervous balance, found on inquiry that 

 the parents were cousins. There is no doubt that in-breeding 

 tends to aggravate to an unexpected degree any nervous 

 tendency that may be present in the parents. 



Turning to the palasontological side of the question, I will 

 first give some of the inferences that may be drawn from it, 

 and afterwards rapidly pass through some of the more strik- 

 ing cases that I have noticed as regards range in time and 

 other points that would lead to such inferences in the 

 different groups of animals in geological history. 



In the first place, the lowly-organised groups have per- 

 sisted in spite of the gradual evolution of more and more 

 highly-organised forms, and that this must be due in large 

 measure to their rapid growth and reproductive powers. 



(2) That groups appear to have a shorter range in time as 

 they acquire a higher degree of organisation. 



(3) That living forms of groups that are dominant at the 

 present time rarely show ancestors of such great specialisa- 

 tion as themselves. 



