Hyatt.J 80 [April 5, 
those animals wlii.'h oftoii present retrogression tlii'ougli sii])i)res- 
sion of ancestral eharacters in the young, althongli their adults are 
perfectly normal aiul perhaps progressive. Consequently ])alingen- 
esis and cenogenesis are, from my point of view, simply different 
forms of tachygenesis, and there is no boundary or distinction 
between tliem. In other words retardation or retrogression occurs 
because of the direct action of tachygenesis upon more suitable 
and more recentl}'' acquired characteristics which are driven back 
upon and may directly replace certain of the ancestral characters 
causing them to disappear from ontogenetic development. •* 
The law of tachygenesis as defined by the writer acts upon all 
characteristics alike and is manifested in genetically connected 
phyla by an increasing tendency to concentrate the characteristics 
of lower, simpler, or earlier occurring, genetically connected forms 
in the younger stages of the higher, more complicated or more spe- 
cialized, or later occurring forms of every grade, whether the 
characteristics arise in adults or in the younger stages of growth. 
Since myfii-st publication in 1866 the law has become clearer to 
me, but I have made no fundamental change in the conception. 
The application of the law to degenerative characteristics appears 
1 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 tiirown back in their development. There is, 
however, another way of formulating the expression for 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 cui^s above the gum until a 
person is past lifty 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? 
