564 TJic Avicrican Naturalist. [July, 
We further observe that no variations of this class occur without 
the antecedent operation of these reactions ; the working hypo- 
thesis thus stands the test of prediction. 4. We accept this 
invariable sequence of race adaptation upon individual adapta- 
tion as proof of a causal relationship. 
6. I admit that this proof may be invalidated in several ways : 
I. By showing in more extended research that these observations 
of sequence are inaccurate or offset by others in which there is no 
such sequence. 2. By showing that the Lamarckian principle, 
while explaining some of the variations of this class, is directly 
contradictory to others. 3. By showing that all these phenom- 
ena may be explained equally well or better by natural selection. 
4. By proving, independently, that the transmission of acquired 
characters never occurs. 
I will now consider each of these cases : 
First. — As regards these observations. They may be examined 
in detail in the studies of Cope, Wortman, or Ryder, and in a 
paper I presented to this Association last year. As the question 
of transmission has been generally assumed in the foregoing 
studies, I think it is now important to review the whole field, 
searching for facts which look against the Lamarckian principle, 
for as we have been hitherto studying with a bias in favor of it, 
some such adverse points may have been overlooked. At 
present, however, I can recall only a single adverse observation, 
that is, in the development of one of the upper cusps, the lower 
cusp which opposes it, and which is therefore supposed to stimu- 
late this development, is found to recede. I have no doubt others 
will be found presenting similar difficulties. 
Second. — As regards the Lamarckian pri?iciple. Several objec- 
tions to the special application of this principle to the evolution of 
the teeth have been raised by Mr. E. B. Poulton : 
A. — To the objection that the teeth are entirely formed before 
piercing the gum, and that use produces an actual loss of tissue 
as contra-sted with the growth of bone, it may be said that by our 
theory it is not the growth itself, but the reactions which produce 
this growth in the living tissue, which we suppose to be trans- 
