448 Duplicate Twins and Doiible Monsters 



divided half) there has been a force or mechanisjn suffi-ciently simiJaf 

 to that contained in the other to cause the main lines, the areas and the 

 patterns to develop as practical duplicates, it has attempted no control in 

 the formation of the separate ridges, and thai these latter, therefore, have 

 developed in obedience to force'? which have appeared later in the develop- 

 ment, and which did not exist in the first germ-nucleus. 



Some explanation of these later forces and their method of action has 

 been given through the recent investigations of Miss Whipple, in the 

 work referred to in the bibliography and now in press. The author has 

 studied the genesis of ridges, both phylogenetically and ontogenetically, 

 in the various orders of mammals, and shows that they are formed from 

 either (1) the coalescence of separate epidermic units, each with a sweat 

 gland (and, typically, a sebaceous gland and a hair), which arrange 

 themselves in rows and form single ridges, or, in other cases, from 

 (2) epidermic nngs, formed by a coalescence of the primary units in 

 circles, which, by becoming elliptical and arranging themselves in rows 

 corresponding to their longitudinal axes, form simultaneously two rows 

 of ridges/ This process may be easily seen, in all stages of transition, 

 in many mammals, especially Marsupials, Lemurs and the lower Pri- 

 mates, along the borders of ridged areas which are surrounded by either 

 the simple units or by the rings, and in all of these lower forms, in which 

 the friction ridges and the pads are still of functional importance, the 

 separate units seemingly arrange themselves in obedience to purely 

 mechanical laws, as though determined through use-inheritance. In 

 fact, I do not know any instance which convinces me so completely of 

 preformation in the egg as the comparison of duplicate twins, nor, on 

 the other hand, one that forms so good an argument in favor of the 

 doctrine of use-inheritance as these investigations of Miss Whipple. The 

 facts and principles thus far brought out in the general study of the sur- 

 face structures of the mammalian chiridium point more and more to 

 the great importance of these parts as a basis for the study of funda- 

 mental biological problems. 



^ This development of separate epidermic units into friction ridges, as thus 

 far ascertained, is a phylogenetic development, as traced by the comparison 

 of adult forms. How much of this may be recapitulated in individual ontog- 

 eny has not as yet been ascertained, but it seems probable that more is left 

 the individual to accomplish in such lower mammals as Marsupials and 

 Lemurs than in the higher primates. Finger patterns which would proba- 

 bly have become the adult form are demonstrable in a simian embryo of 

 70-90 days. 



