ASPECTS OF BIOLOGY AND METHOD OF BIOLOGICAL STUDY. 39 
with the phenomena of individual development. Now such Plu- 
mularida as have been followed in their development from the egg 
to the adult state do actually present well-developed nematophores 
before they show a trace of hydrothecæ, thus passing in the course 
of their embryological development through the condition of a 
graptolite, and recapitulating within a few days stages which it 
took incalculable ages to bring about in the paleontological devel- 
opment of the tribe. 
I have thus dwelt at some length on the doctrine of evolution þè- 
cause it has given a new direction to biological study and must 
powerfully influence all future researches. Evolution is the high- 
est expression of the fundamental principles established by Mr. 
Darwin, and depends on the two admitted faculties of living be- 
ings— heredity, or the transmission of characters from the parent 
to the offspring; and adaptivity, or the capacity of having these 
characters more or less modified in the offspring by external agen- 
cies, or it may be by spontaneous tendency to variation. 
The hypothesis of evolution may not, it is true, be yet estab- 
lished on so sure a basis as to command instantaneous acceptance ; 
and for a generalization of such vast significance no one can be’ 
blamed for demanding for it a broad and indisputable foundation 
of facts. Whether, however, wedo or do not accept it as firmly 
established, it is at all events certain that it embraces a greater 
number of phenomena and suggests a more satisfactory explana- 
tion of them than any other hypothesis which has yet been 
proposed. 3 
With all our admiration, however, for the doctrine of evolution 
as one of the most fertile and comprehensive of philosophic hy- 
potheses, we cannot shut our eyes to the difficulties which lie in the 
way of accepting it to the full extent which has been sometimes 
claimed for it. It must be borne in mind that though among some 
of the higher vertebrata we can trace back for some distance in 
geological time a continuous series of forms which may safely be 
regarded as derived from one another by gradual modification — as 
has been done, for example, so successfully by Prof. Huxley in the 
case of the horse—yet the instances are very few in which such a 
sequence has been actually established ; while the first appearance 
in the earth’s crust of the various classes presents itself in forms 
which by no means belong to the lowest or most generalized of 
their living representatives. On this last fact, however, I do not 
