478 The American Naturalist. [June, 



should decrease as the extent of time elapsing since the struc- 

 ture disappeared increases. This law is apparently established 

 in the case of the condylar and intercondylar foramina, and 

 if we examine all the percentages which have been estab- 

 lished, we see at once that they bear a ratio to time ; compare 

 the relative frequency of the ischio-pubic (fifty per cent.), 

 dorso-epitrochlearis (five per cent,), and levator-claviculse 

 (1.66 per cent.) muscles with the periods which have elapsed 

 since their past service. This is why it is so important to 

 establish percentages for all our atavistic organs ; fuller statis- 

 tics will not only bear upon heredity, but I can conceive of 

 their application to the extremely difficult problem of estima- 

 ting geological time. We must, of course, establish as a 

 standard cases of congenital variation in which the frequency 

 of recurrence has been steadily declining in the same race 

 between two known periods of time — an available structure is 

 the intercondylar foramen or supratrochlear foramen, as 

 recorded by Blanchard, Shepherd and others. 



The reversional tendency is hereditary. There are many 

 cases, both of reversions (as in the teeth) and indefinite varia- 

 tions being hereditary, that is, reappearing in several genera- 

 tions, or skipping a generation and recurring in the second. 



Summary. — There are clearly marked out several regions 

 in the human body in which evolution is relatively most 

 rapid, such as the lower portion of the chest, the upper cervi- 

 cals, the shoulder girdle in its relation 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 adapta- 

 tion 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 parts of the body are not equally 

 variable, but these centers of evolution are also the chief cen- 

 ters of variability. The variations here are not exclusively, 

 but mainly, of one kind ; they rise from the constant 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 a new one ; it maintains its new func- 



