1897.] A Chapter in the History of Science. 855 
Second, that of exact parallelism between the adult of one individual or 
set of individuals and a transitional stage of one or more other individ- 
uals. This doctrine is distinct from that of inexact parallelism which had 
already been stated by von Baer. And that this law expresses the origin 
of genera and higher groups, because, 
Third, they can only be distinguished by single characters when all their 
representatives come to be known 
Fourth, that genera and various other groups have descended, not 
from a single generalized genus, etc., of the same group, but from cor- 
responding genera of one or more othergroups. This was called the doc- 
trine of homologous groups. 
Fifth, the doctrine that these homologous groups belong to different 
geological periods, and, 
Sixth, to different geographical areas, which, therefore, in some instan- 
ces, are, 
Seventh, related to each other in a successional way like the epochs of 
geological time. 
f these doctrines it may be observed that the first and second are now 
the common property of evolutionists, and are recognized everywhere as 
matter of fact. The names which I selected to express them have, how- 
ever, only come into partial use. The author believes that, although the 
doctrine was vaguely shadowed out in the minds of students prior to the 
publication of this essay, it had not previously been clearly expressed, 
nor been reduced to a demonstration. Of the truth of the doctrine the 
author is more than ever convinced, and he believes that paleontological 
discovery has demonstrated it in many instances, and that other demon- 
Stratious will follow. The fonsth Propestion (that of homologous groups) 
is now held as a hypothesis explaining the phylogeny of various groups of 
animals. For the descent of one homologous group from another, the 
term polyphyletic has been coined. It remains to be seen whether the 
doctrine is of universal application or not. That homologous groups be- 
long to different geological horizons, as stated under the fifth head, has 
been frequently demonstrated sincs the publication of the essay. That 
the sixth proposition is true in a certain number of cases is well known, 
and it follows that the seventh proposition is also true in those cases. 
The latter hypothesss, which was originally advanced by Professor Agas- 
8iz, is, however, only partially true, and the advance of paleontological 
study has not demonstrated that it has had a very wide application in 
geological time. 
A proposition which was made prominent in this essay was that the - 
prevalence of non-adaptive characters in animals proves the inadequacy 
of hypotheses which ascribe the survival of types to their superior adap- 
tion to their environment. Numerous facts of this kind undoubtedly in- 
