1896.] Lost Characteristics. 15 



The law of tachygenesis as defined by the writer acts upon 

 all characteristics and tendencies alike, and is manifested in 

 genetically connected phyla by an increasing tendency to con- 

 concentrate the characteristics of lower, simpler, or earlier oc- 

 curring, genetically connected forms in the younger stages of 

 the higher, more complicated or more specialized, or more de- 

 graded, or later occurring forms of every grade, whether the 

 characteristics arise in adults or in the younger stages of growth. 

 Since my first publication in 1866, the law has become clearer 

 to me, but I have made no fundamental change in the con- 

 ception. The application of the law to degenerative character- 

 istics appears to me to explain why there are degenerative forms 

 in the phylum which are indicated by the senile stages of the 

 individual. 



The degenerative changes of the senile period may, and 

 practically in all cases do, tend to the loss of characteristics of 

 the adult period and consequently in extreme cases bring 

 about not only the loss of a large proportion of progressive 

 characteristics, but loss in actual bulk of the body as compared 

 with adults, as has been stated above. This is usually re- 

 garded as due to the failure of the digestive organs or defective 

 nutrition, and this may be true in many examples ; but, on 

 the other hand, it often begins in individuals long before there 

 is any perceptible diminution in size, and may occur in dwarfs 

 and in some degenerate species in the early stages, and finally 

 in series of species according to the law of tachygenesis, so that 



kind? That is to say, do not the parts and characters show a tendency to disap- 

 pear earlier and earlier, and are they not, in most cases, at the time of disappear- 

 n earlier stages of growth than that in which they c 



e due primarily to the fact that the 

 ing to disappear from disuse, tie . and -ecoiularily to their internal position. 

 When they cease to be ableto breakthrough the gum, will they not still continue 

 to develop at the same stage as the other teeth, and will not their rudiments be 

 likely tobe present at this early stage long after they have ceased developing into 



