Hyatt.] 
76 [April s. 
I have referred above to classes of morphic characteristics tliat 
cannot be considered as based upon increments of growth, but there 
are also others whicli are important for a clear conception of 
genesiology and bioplastology. Minot's researches enable one to 
see clearly that the reduction of parts or characteristics which takes 
place through the action of the law known as the law of accelera- 
tion in development (often also descriptively mentioned as abbre- 
viated or concentrated development) cannot be considered as due 
to growth. 
It seems probable from my own researches pubhshed in various 
communications, but more especially in the "Genesis of the 
Arietidae,"^ that the action in this case is a mechanical replacement 
of the earlier and less useful ancestral characteristics and even parts 
by those that have arisen later in the history of the group. We can 
fully understand the phenomena of acceleration in development 
only when Ave begin by assuming that the characteristics last 
introduced in the history of any type were more suitable to the 
new conditions of life on the horizon of occurrence of the species 
than those which characterized the same stock when living on 
preceding horizons. These new characters would necessarily, on 
account of their greater usefulness and superior adaptability, 
ultimately interfere with the development of the less useful 
ancestral stages and thus tend to replace them. The necessary 
corollary of this jirocess Avould be the acceleration or earlier 
appearance of the ancestral stages in direct proportion to the 
number of new characteristics successively introduced into the 
direct line of modification during the evolution of a group. 
If this be true it can hardly be assumed that the loss of charac- 
teristics and parts taking place in this way is directly due to 
growth force. If growth has anything to do with these phenomena 
it must act indirectly and, as in the repetition of other similarities 
and parallelisms, under the guidance of heredity. 
This law since its first discovery has received verification 
from a number of sources and is fairly entitled to rank as a 
fundamental law of genesiology. 
The phenomena of development which have been observed as 
the manifest results of the action of this law have been called by 
I Siiiitlisoniaii criiitrilnitions to knowledge, v. 26, p. -10-48, 1889; also Mem. nius, 
comp. zool., V. 16, 18811. 
