376 cook 



a kind of watered silk appearance, as a result of the deeper 

 black of the spots, which, though invisible, were really there 

 just the same." l 



In a similar case of supersexual dichromatism in a chrysomelid 

 beetle experiments showed that the two color-forms could be 

 separated and established as uniform varieties by selective 

 breeding. 2 The mating of black individuals produced only 

 black offspring in the first generation, while matings of spotted 

 individuals continued to give a proportion of black offspring 

 until the third generation. 



SPECIFIC CONSTITUTIONS MODIFIED BY RESTRICTED DESCENT. 



This is the second form of diversity of constitutions revealed 

 by cross-sections of networks of descent. Unlike the specializa- 

 tions of heterism, the members of groups formed by restricted 

 descent do not, of course, breed together, for it is in this 

 that the restriction of descent consists. The specializations of 

 heterism are in accord with the evolutionary advancement of the 

 species, but the groups formed by restricted descent are removed 

 from the conditions of free interbreeding and of normal evolu- 

 tionary progress. They represent, instead, the different stages 

 of a process of deterioration. 



Symbasic Species. — Species with descent unrestricted, con- 

 sisting of large numbers of diverse individuals freely inter- 

 breeding in a broad, continuous and regular network of descent. 



A species is not merely an aggregation of organisms, whether 

 alike or different ; the organisms are connected by a completely 

 interwoven fabric of lines of descent. Such plants as Portulaca 

 oleracea, Poa pratensis and Ceratodon purpureus, may serve 

 as examples of very widely distributed symbasic species. 



Porric Species. — Species made up of partially segregated 

 subspecies. The cross-section of the network of descent, instead 

 of showing a rounded or regular form, is irregular, or partially 

 subdivided into arms or branches. 



Widely distributed species, but locally diversified, like the 



1 Whitney, Caspar, 1904. Outing for April, p. 14. 



2 McCracken, I., 1905. A study of the Inheritance of Dichromatism in Lina 

 Lapponica. Journal of Experimental Zoology, 2 : 117. 



