GENERAL REMARKS. 23 
As a rule, among Ammonitinz the reverse is the case, and the disap- 
pearance of characters takes place through the earlier and earlier develop- 
ment and shorter and shorter life history of each characteristic, or through 
tachygenesis, as stated and illustrated in many of my papers. Here I have 
space only for one quotation, which will serve to show my meaning more 
plainly: 
Specialization by reduction of parts is evidently included under the head of 
retardation by Cope; thus in Origin of the Fittest (p. 353), he says that ‘*change of 
structure during growth is accomplished either by addition of parts (acceleration) or 
by subtraction of parts (retardation).” So far as my experience goes, in the major 
number of cases the parts or characters that are undergoing reduction disappear 
according to the law of tachygenesis. They reappear in the ontogeny at earlier 
and earlier stages, or exhibit this tendency in the same way as characters of the. 
progressive class, but their development is not so complete as in ancestral forms. In 
this sense they can be regarded as retarded or thrown back in their development. 
There is, however, another way of formulating the expression of this. Instead of 
regarding this disappearance by retrogressive gradations as due to a tendency 
opposed to acceleration, is it not a tendency of the same kind? That is to say, do 
not the parts and characters show a tendency to disappear earlier and earlier, and are 
they not in most cases at the time of disappearance present only in earlier stages of 
growth than that in which they originated in ancestral forms? 
Is not the case of the wisdom teeth exceptional? The frequently extremely late 
external appearance of these is not accompanied by a later origin of their rudiments 
in the jaw. Although they may not appear in many cases above the gum until a 
person is past fifty, is not this real retardation due primarily to the fact that they are 
deficient in growth power (tending to disappear from disuse, etc.), and secondarily to 
their internal position? When they cease to be able to break through the gum will 
they not still continue to develop at the same stage as the other teeth, and will not 
their rudiments be likely to be present at this early stage long after they have ceased 
developing into perfect teeth? 4 
The whalebone whales are examples of this kind so far as the teeth are 
concerned, although here the disappearance is correlated with the develop- 
ment of whalebone from the rugze of the roof of the mouth. Nevertheless 
the suppression of the teeth in full-grown animals does not take place by 
later and later development, and the rudiments of the teeth are still present 
in the early stages. 
4 Bioplastology: Proc. Boston Soc. Nat. Hist., Vol. XX VI, note to p. 80. 
