j>4 BIOLOGICAL SOCIETY OF WASHINGTON. 



act nature of this law be worked out, and that all attempts to 

 popularize it be correct and be accompanied by the necessary 

 qualifications and an explanation of important subordinate 

 laws. Only thus can the coarse and repugnant conceptions 

 which seem to be taking possession of the popular mind be 

 removed. 



EXTINCTION OF TRUNK LINES OF DESCENT. 



It is especially important that the first great qualifying prin 

 ciple, which I propose to call the law of the extinction of trunk 

 lines of descent, be made clear, since it lops off at one stroke, 

 the most serious of all popular misconceptions. I shall assume 

 that the principle of genealogic dichotomy is clear to the minds 

 of all, since it is nothing more than the simple law of toco- 

 gonic descent as exemplified in every human genealogy and 

 every family register applied to all life, except that it relates to 

 species instead of individuals. 



Sympodial Dichotomy. But while organic phylogeny is, in 

 a certain sense, arborescent and dichotomous it cannot be 

 directly compared to any ordinary tree nor even to a plant that 

 branches in a strictly forking or dichotomous manner, such as 

 an Any chia, for example. It resembles more nearly that form 

 of indeterminate growth which is termed sympodial, in which, 

 instead of the two forks being equal and divergent, one of 

 them has to be regarded as the main trunk and the other as a 

 branch, but in which the branch possesses the greater vigor 

 and vitality and virtually becomes the main trunk, the true 

 stem dwindling, and either dying out entirely or continuing as 

 a reduced and degenerate form. There are many plants, such 

 as the common grape-vine, the houseleek, the heliotrope, and 

 the for-get-me-not, that exhibit this sympodial dichotomy. 



