Different Kinds of Selection 137 



grasses, that are not wanted. This is an illustration 

 of lethal selection, a process of pruning. But it is 

 possible to work towards the perfect lawn in another 

 way, by using a differential fertilizer. This promotes 

 the growth and multiplication of the grass, but has 

 no such influence on the weeds. The increase of the 

 grass tends to smother the weeds. This is an illustra- 

 tion of what is called reproductive selection. The 

 forms with the advantageous character may be helped 

 to success if they have a fertility much greater than 

 that of those that have not the character in question. 

 In many cases lethal and reproductive selection may 

 both be operative. There are other kinds of selection, 

 such as germinal selection and sexual selection, but 

 the present point is simply that while the logic or 

 principle of the sifting remains the same, the modes 

 of operation may be diverse. 



The sifting will vary according to the nature of 

 the "struggle," whether it is between fellows of the 

 same kith and kin, or between foes quite different in 

 nature; or it may be non-competitive — the organism 

 pitting itself against the callous physical fates. Or 

 the sifting will vary according to the need which 

 brings about the clash. It may be for food, for foot- 

 hold, for a place in the sun, for a mate or for a harem, 

 or for room for a cradle. The struggle may be for 

 luxuries; it may be for self-satisfaction and self- 

 preservation, or it may be concerned with the welfare 

 of the offspring. Even among animals it may rise into 

 an endeavor after well-being. 



It is interesting to try to follow the changes in the 



