1880.] A Review of the Modern Doctrine of Evolution. 167 
I. Systematic relation (taxonomy); 2. Embryonic growth (em- 
bryology); 3. In geologic time (paleontology); 4. And in the 
coincidence in the successions seen in Nos. I, 2 and 3. 
The fact that it is necessary to arrange animals in an order cor- 
responding with the phases of their embryonic history is remark- 
able; but the further fact, shown by paleontology, that the same 
Succession marked the ages of past time, at once brings evolution 
within the limits of strong probability. Nevertheless, all this 
might have been a mere system, without transitions between its 
members; organic types might have been created unchangeable, 
but presenting the mutual relations in question. But if transi- 
tions among these members can be shown to take place, then 
indeed the phenomena mentioned receive a sufficient explanation. 
They are seen to be the necessary relations of the parts of a shift- 
ing scene of progression and retrogression; they express com- 
binations of structure, which, though often long enduring, are, 
nevertheless, not perpetual, but give way to other combinations 
to be in their turn dissolved. Now, if there is anything well 
known in nature, it is that there are divisions of various ranks in 
the vegetabie and animal kingdoms, whose contents present varia- 
tions of structure which are confessedly additions to or subtrac- 
tions from the characters of ancestors, which have appeared during 
ordinary descent. The protean species, genera, etc., are well 
known to biologists, and every naturalist who admits varieties, 
sub-species, sub-genera, etc., admits derivation so far as they are 
concerned. The facts of variation, including “ sporting,” etc., are 
notorious, not only among domesticated, but also in wild animals 
and plants. The facts have led some persons to suggest that 
Species have been produced by evolution from a single specific 
center, but that the genus and other comprehénsive divisions are 
unchangeable. But I think I have shown, ina paper entitled, 
“The Origin of Genera,” that the structural characters which 
define genera, and even higher divisions, are subjects of variation 
to as great an extent as are the less profound specific characters; _ 
and, moreover, that the evidence of derivation which they present 
's singularly clear and conclusive. The changes of both genus 
and species character are always of the nature of additions to or 
Subtractions from those of one generation displayed by their 
descendants. As such, they form the closing chapters of the 
embryonic or growth-history of the modified generation. — — — — 
"Philadelphia, 1869, “ Proceedings Academy Natural Sciences, 1868.” _ ae 
