1909] LEAVITT—HOMOEOSIS IN PLANTS 65 



at the present juncture point a way to the correct mechanical ex- 

 planation of development, possess in this connection a considerable 

 prospective value. An adequate theory of ontogenesis must take 

 these facts into account. 



Their value lies in their exceptionality. Homoeotic occurrences 

 are fundamentally antithetic to our usual conception of the method 

 of development. The most general and fundamental of our notions 

 of a mechanically autonomic evolution from egg to adult, is that of 

 a series of states (A, B, C, D .... X, Y, Z), so related that each 

 one necessarily serves as the stage upon which the next state arises. 

 Every one of the several states seems to be the needful condition for 

 the appearance of the next following. This is the primary concep- 

 tion of ontogenesis derived from ordinary experience. It is contra- 

 dicted by homoeotic formations. We see the usual sequence violated 

 at some point, and a state (as X) arising upon a state (as D) from 

 which it does not normally arise, and from which we have supposed 

 that, in the very nature of the process, it cannot arise directly. 



in homoeosis, then, we have a new ontogenetic phenomenon. 

 Herein lies its worth; for every datum of a new sort adds to the 

 materials for a true theory of development, and increases the chances 

 of our finding a clew to the right construction and combination of 

 the materials. 



Although the times are doubtless not ripe for the appearance of 



an adequate theory, and further attempts in this direction may be 



Profitless for the present, yet almost universally there has been felt 



e desirability of biomechanical hypotheses going behind the bare 



^cte of development. We have had from Darwin, Nageli, 



ies, Sachs, Roux, Weismann, Driesch, and others a series 

 ^ more or less elaborate attempts at an explanation. I think that 



nSTh ^ dl familiar with this line of biologic endeavor will recog- 



enTe f Pr ° 8reSS in subst antial knowledge effected under the influ- 



e such speculations— even though they deal with gemmules, 



bevond\ miCdlae ' biophores ' and less concrete "organizations," far 

 somTof * h ran§e ° f the microsco P e - And k is entirely possible that 

 wl»vu su gg e stions already offered may be germinal points from 



*£ a verifiable system will develop. 



6 m ° st P ersi stent attempt at a solution has been made by Weis- 



