1876.] Discovery of the Laws of Evolution. 219 
the same time. Thus Lamarck found the variations of species 
to be primary evidence of evolution by descent. Darwin enun- 
ciated the law of “ natural selection ” as a result of the struggle 
for existence, in accordance with which “ the fittest only survive.” 
This law, now generally accepted, is Darwin’s principal contri- 
bution to the doctrine. It, however, has a secondary position in 
relation to the origin of variation, which Lamarck saw, but did 
not account for, and which Darwin has to assume in order to 
have materials from which a “ natural selection ” can be made. 
The relations exhibited by fully grown animals and plants 
with transitional or embryonic stages of other animals and plants 
had attracted the attention of anatomists at the time of Lamarck. 
Some naturalists deduced from this now universally observed 
phenomenon that the lower types of animals were merely re- 
pressed conditions of the higher, or, in other words, were embry- 
onic stages become permanent. But the resemblance does not 
usually extend to the entire organism, and the parallels are so 
incomplete that this view of the matter was clearly defective, and 
did not constitute an explanation. Some embryologists, as Lere- 
boullet and Agassiz, asserted that no argument for a doctrine of 
descent could be drawn from such facts. 
; The speaker, not adopting either view, made a full investigation 
into the later embryonic stages, chiefly of the skeleton of the batra- 
chia, in 1865, and Professor Hyatt, of Salem, Mass., at the same 
time made similar studies in the development of the ammonites 
and nautili. The results, as bearing on the doctrine of evolu- 
tion, were published in 1869 in a paper entitled The Origin of 
Genera. (Proceedings of the Philadelphia Academy of Natural 
Sciences.) The relation usually observed between adult types 
and transitional stages was there termed inexact parallelism. 
It was then pointed out that the most nearly related forms of 
animals do present a relation of repression and advance, or of 
Permanent embryonic and adult type, leaving no doubt that the 
one descended from the other. This relation was termed exact 
Parallelism. Tt was also shown that if the embryonic form were 
the parent, the advanced descendant was produced by an in- 
creased rate of growth, which phenomenon was called accelera- 
tion s but that if the embryonic type were the offspring, then its 
failure to attain to the condition of the parent is due to the super- 
vention of a slower rate of growth. To this phenomenon the 
term retardation was applied. It was then shown that the inexact 
_ Parallelism is the result of unequal acceleration or retardation ; 
