586 NATURAL SCIENCE. ocr.. 



limited to at most two kinds of individuals — males and females : 

 the case of polymorphic species in which some individuals are in- 

 capable of taking any part in reproduction has now to be considered. 

 The cases of alternation of sexual with asexual, and of dicecious with 

 hermaphrodite or parthenogenetic generations will need no special 

 elucidation, for what has already been said and what has now to be 

 said of the Termites, etc., will, with only slight alterations, apply to 

 them also. 



In non-social species such as birds, where the society, if it. exists 

 at all, includes at most two adults, the males and females alone do 

 all the work, and any other individuals, if produced at all, would not 

 only be useless competitors for food and doomed to die without 

 offspring, but would also be a disadvantage to the stock. More 

 accurately, that stock which, by virtue of its constitution, prodviced 

 sterile individuals would thereby be at a disadvantage in the struggle 

 fov perpetuation. Such individuals are occasionally produced, but 

 natural selection weeds out the stocks producing them frequently in 

 the same way as it weeds out all stocks hampered by any other 

 disadvantage whatever : and hence the percentage of " neuters " in 

 non-social species is almost infinitesimal. 



Among social species it is otherwise. Where all individuals 

 work for the common good, the production of neuters need not be a 

 disadvantage, and it may even be an advantage. The continuous 

 labour of such members, uninterrupted and unhampered by repiro- 

 ductive functions, is of great value to a community ; and the stock 

 which, by virtue of its constitution, produces individuals either varying 

 in reproductive capacity, or liable, under certain conditions as to 

 diet, so to vary, is at an advantage as compared with other social 

 stocks whose constitution determines the production of fertile 

 offspring only. Hence natural selection will tend, within due limits, 

 to progressively increase the average percentage of neuters (or 

 potential neuters) among the individuals of successive generations. 

 It is not the individuals which are selected, but the stocks. The unit, 

 corresponding to the individual or a pair in an ordinary species, is 

 in these species the whole society. The stock which is favoured by 

 natural selection is not that producing the fittest individuals, but that 

 producing the fittest society. The constitutional characters which 

 natural selection favours are those upon which the production of the 

 fittest society depends. What society is the fittest depends on the 

 environment of that society in part, but largely also upon the 

 organisation of the society itself. The environment and social 

 organisation of the Termites, or "white ants," are such, that natural 

 selection has determined a stock-constitution, one peculiarity of 

 which is that, instead of the ordinary two types of offspring (male 

 and female), there are six types. That constiti?tion is kept constant 

 by natural selection, and hence those six types of individuals are found 

 among the offspring, if not of every " queen," at least of every society. 



