330 PRESENT PROBLEMS IN EVOLTTTION AND HEREDITY. 



ar<l cases of congenital variation in wliicli llie fieqnenc^' of recurrence 

 has been steadily declining in the siinie race between two known ])eriods 

 of time — an available structure is the intercondylar foramen or su]n'a- 

 trochlear foramen, as recorded by lUanchard, Shepherd, and others. 



The reversional teii<lency is hereditary. There are nniny cases, both 

 of reversions (as in the teeth) and iiulefinite variations being hereditary, 

 that is, re-appearing in several generations, or skipping a generation 

 and recurring in the second. 



Siiininary. — There are clearly marked out several regions in the human 

 body in Avhich evolution is relatively most rai)id, such as the lower 

 portion of the chest, the upi>er cervicals, the shoulder girdle in its rela- 

 tion to the trunk, the lower portion of the arm and hand, the outer 

 portion of the foot. We notice that these regions especially are centers 

 of adaptation to new habits of life in which new organs and new rela- 

 tions of parts are being acquired and old organs abandoned. 



We observe also that all i)arts of the body are not equally variable, 

 but these centers of evolution are also the chief centers of variability. 

 The variations here are not exclusively, but mainly, of one kind; they 

 rise from the constnnt struggle between adaptation and the force of 

 heredity. Here is a muscle like the extensor indicis attempting to give 

 up an old function and establish anew one; it maintains its new func- 

 tion for several generations, and then goes back without any warning 

 to a function which it had thousands of years ago. Thus the tVu-ce of 

 reversion strikes us as a universal factor. 



l>row the singular fact about reversion is the frequent })roof it affords 

 of what Galton has called "particulate inheritance.'' When the ex- 

 tensor indicis reverts, all the muscles around it may be nonnal; there- 

 fore we are obliged to consider each of these muscles as a structure by 

 itself, with its own particular history and its own tendencies to deveh)X5 

 or degenerate. Thus it is misleading to base our theory of evolution 

 and heredity solely upon entire organs; in the hand and foot we have 

 mimerous cases of nuiscles in close <'ontiguity, one steadily develo])ing, 

 the other steadily degenerating, lleversion very rarely acts upon many 

 structures at once; when it does, we have a case of diffused anomaly, 

 some repetition in the epidermis, or in the entire organism of a 

 lower type. 



Yet in spite of reversion and the strong force of repetition in inherit- 

 .ance, the human race is steadily evolving into a new type. We must, 

 it seems to me, admit that an active principle is constantly operating 

 upon these particular structures, guiding them into new lines of adapta- 

 tion, acting upon widely separate minor parts, or causing two parts, 

 side by side, to evolve in o]>posite directions, one coward degeneration, 

 the other toward development. 



1 may now recall the two opposed theories as to what this a«'tive 

 principle is : 



The first, and oldest, is that individual adaptation, or the tendencies 



