1889] Transmission of Acquired Characters. 563 
strictly to the lines of use and disuse. The indirect proof is that 
the natural selection of chance variations is unsupported by ob- 
servation and is inadequate to explain the variation phenomena 
of the second class. 
4- I will first briefly consider the former. The distinctive 
feature of palaeontological evidence is that it covers the entire 
pedigree of variations, the rise of useful structures not only from 
their minute, apparently useful condition, but from the period 
before they appear. The teeth of the mammalia render us the 
most direct service, as compared with the feet, since they furnish 
not only the most interesting correlations and readjustments, but 
the successive addition of new elements. With a few exceptions 
■which need not be noted here, all the mammalia started with 
teeth of the simple conical type — like the simple cusps of rep- 
tiles. Practically every stage between this single cusp and the 
elaborate multicusped recent molars is now known. Every one 
of the six main cusps of the molar of Hyracotherium, for ex- 
ample, a type of an important central stage in the ungulate 
dentition, is first indicated at the first point of contact or ex- 
treme wear between the upper and lower molars ; this point of 
wear is replaced by a minute tubercle, which grows into a 
prominent cusp. These are the laws of cu.sp development, as 
observed in every known phylum of mammalia : 
I- — The primary cusps first appear as cuspules, or minute 
cones, at the first points of contact between the upper and 
lower molars in the vertical motions of the jaws. 
II. — The modeling of cusps into new forms, and the acquisition 
of secondary position, is a concomitant of interference in the 
horizontal motions of the jaws, 
5- The evidence, of which this is only a single illustration, 
has accumulated very slowly. The line of reasoning from this 
particular series of observations is as follows : i . The new main 
variations, in the teeth and skeleton of every complete series, are 
observed to follow certain definite purposive lines. 2. By careful 
analysis of the reactions to environment which would occur in 
the individuals by the laws of growth, we observe that the race 
variations strictly conform to the line of these 
