ASPECTS OF KINETIC EVOLUTION 273 



the members of the breed, those not conforming to the approved 

 standard being ruthlessly weeded out. The type having been 

 once established by this means, the variety remains for a period 

 of years more or less uniform, generally very much more so 

 than the members of wild species in nature. It is the experience 

 of all history, however, that varieties decline after a time from 

 their original excellence and have to be replaced by other, newer 

 sorts, which by reason of their more recent origin have been 

 subjected to shorter periods of inbreeding. The degeneration 

 of the older variety may be indicated in a number of ways, such 

 as a decline in fertility or weaker vegetative growth, or suscep- 

 tibility to fungous and insect parasites, so that it usually dis- 

 appears from cultivation or husbandry before the final stage of 

 sterility and extinction is reached, though the tendency in this 

 direction often becomes very obvious. 



One of the symptoms of degeneration is the appearance of 

 numbers of freaks, sports or mutations, as they are variously 

 called. These variations of domesticated plants and animals 

 are often interesting, and sometimes valuable on account of 

 some special peculiarity, such as long hair, double flowers, 

 albino color, etc. This is especially true among the plants 

 cultivated for their flowers, where the never-ending diversity of 

 garden varieties is obtained by the preservation of the numerous 

 mutations into which wild species commonly "break" after a 

 period of domestication and inbreeding. 



A general tendency among all such sorts is towards lessening 

 of seed production, and finally complete sterility may ensue. 

 The last is not a calamity in species which can be propagated 

 by cuttings, and many of our cultivated species have reached 

 this condition. With others, as for example, the " seedless " 

 green-house or forcing cucumbers, the extreme scarcity of seeds 

 which renders the variety desirable is at the same time a serious 

 obstacle to its cultivation. 



On the strength of the older static, uniformitarian theory of 

 life, some writers have insisted that mutations must be caused 

 by environment, there being, in their opinion, nothing else to 

 cause them. The diversity of the mutations could be explained, 

 under this doctrine, only by environmental differences, such as 



