ASPECTS OF KINETIC EVOLUTION 247 



Likewise with plants and animals ; it is necessary only to 

 become personally acquainted with them to appreciate their 

 individual differences. The shepherd knows all his sheep as 

 individuals, also the poultry-raiser knows the eggs of the indi- 

 vidual hens, and the farm boy knows the kind of nuts which 

 each hickory tree produces. 



An instructive instance of natural heterism was observed in a 

 species of agave which is extremely abundant on the mountains 

 to the north of Chiantla, in the department of Huehuetenango, 

 Guatemala. The size, shape, color and spine-development of 

 plants growing by the hundreds along the roadside varied end- 

 lessly. Some were pale-green and heavily pruinose, some slightly 

 pruinose and much darker green. Some tapered rather gradu- 

 ally to the point, some carried their width to near the end. On 

 some the spines were very numerous and prominent, on others 

 scattering and small, and with all grades and combinations of 

 these and other varying characters. It is not claimed that these 

 agaves have essentially greater individual differences than other 

 plants. The phenomenon of heterism is rendered unusually 

 striking because their large leaves have a very definite form and 

 are closely alike on the same plant, and thus give unusually 

 favorable opportunities for observing and comparing the differ- 

 ences which exist. 



SPECIALIZATIONS OF HETERISM. 



The recognition of the facts of heterism, the existence of 

 intraspecific diversity for its own sake, and of its own physio- 

 logical value to the species might appear to rest on merely theo- 

 retical ground were it not for the many specializations of heterism 

 for which no use or meaning has even been imagined, other than 

 that of maintaining a desirable diversity of descent. 



In some species heterism has remained unspecialized. The 

 individuals are different, but still all equivalent and alike, pos- 

 sessing all the essential vegetative and reproductive parts. Such 

 species secure the benefits of heterism only by the introduction 

 of new characters, for each character can be shared ultimately 

 by all the members of the species and thus ceases to be of value 

 as a means of maintaining diversity of descent. 

 Proc. Wash. Acad. Sci., December, 1906. 



