424 
foliaceous appendages or stipules, which are placed at the base 
of the leaf-stalk. Now this typical leaf affords the key to the 
homologies of the tendrils in the two cases under examination. 
Take the Smilax: in this case there are no stipules of the 
ordinary form, but the two tendrils hold exactly the position of 
the stipules in our type-leaf, and must be regarded as repre- 
senting them. _ We have only to imagine these stipules so modi- 
fied in their form as to become reduced to two long spiral 
threads, and we shall at once have the tendrils of the Smilax ; 
on the other hand let the stipules in our type remain as leaf-like 
organs, and let the rest of the leaf—the lamina and petiole—lose 
its normal character, and become changed into a spiral thread, 
and we shall then have the stipules of our type-leaf retained in 
the two opposite leaf-like organs of the Lathyrus, while the 
remainder of the type-leaf will present itself in the condition of 
the Lathyrus tendril which springs from the central point between 
them. 
The tendrils of the Smilax and of the Lathyrus aphaca are 
thus not homologous with one another, but only analogous, 
while those of the Smilax are homologous with a pair of stipules 
and those of the Lathyrus homologous with the lamina and 
petrile of a leaf. 
Besides the homology discoverable between the organs of 
different animals and plants, a similar relation can be traced 
between organs in the same animal or plant ; as, for example, 
that between the different segments of the vertebral column, 
which can be shown'to repeat one another homologically ; and 
that between the parts composing the various verticils of the 
flower and the leaves in the plant. 
The existence of homological relations such as have been just 
illustrated admits of an easy explanation by the application of 
the doctrine of descent, according to which the two organs com- 
pared would originate from a common ancestral form. In accor- 
dance with this hypothesis, homology would mean an identity 
of genesis in two organs, as analogy would mean an identity 
of function. 
Distribution and Evolution” 
Another very important department of biological science is 
that of the Distribution of organised beings. This may be either 
Distribution in Space, Geographical Distribution: or Distri- 
bution in Time, Paleontological Distribution. Both of these 
have of late years acquired increased significance, for we have 
begun to get more distinct glimpses of the laws by which they 
are controlled, of the origin of Faunas and Floras, and of the 
causes which regulate the sequence of life upon theearth. Time, 
however, will not allow me to enter upon this subject as fully 
as its interest and importance would deserve, and a few words 
on Palzontological Distribution is all that I can now venture on. 
The distribution of organised beings in time has lately come 
before us in a new light by the application to it of the hypothesis 
of evolution. According to this hypothesis, the higher groups 
of organised beings now existing on the earth’s surface have 
come down to us with gradually increasing complexity of struc- 
ture by continuous descent from forms of extreme simplicity 
which constituted the earliest life of our planet. 
In almost every group of the animal kingdom the members 
which compose it admit of being arranged in a continuous series 
passing down from more specialised, or higher, to more gene- 
ralised or lower forms; and if we have any record of extinct 
members of the group, the series may be. carried on through 
these. Now while the descent hypothesis obliges us to regard 
the various terms of the series as descended from one another, 
the most generalised forms will be found among the extinct ones, 
and the further back in time we go the simpler do the forms 
become. 
By a comparison of the forms so arranged we obtain as it were 
the law of the series, and can thus form a conception of the 
missing terms and continue the series backwards through time, 
even where no record of the lost forms can be found, until from 
simpler to still simpler terms we at last arrive at the conception 
of a term so generalised that we may regard it as the primordial 
stock, the ancestral form from which all the others have been 
derived by descent. 
This root form is thus not actually observed, but is rather ob- 
tained by a process of deduction, and is therefore hypothetical. 
We shall strengthen, however, its claims to acceptance by the 
application of another principle. The study of embryology 
shows that the higher animals, in the course of their deyelop- 
ment, pass through transitory phases which have much in com- 
mon with the permanent condition of lower members of the 
NATURE 
[ Sept. 18, 1873 
type to which they belong, and therefore with its extinct repre- 
sentatives. We are thus enabled to lay down the further principle 
that the individual, in the course of its own development from 
the egg to the fully formed state, recapitulates within that short 
period of time the various forms which its ancestry presented in’ 
consecutive epochs of the world’s history ; so that if we knew all 
the stages of its individual development, we should have a long 
line of its descent. Through the» hypothesis of evolution, 
palzontology and embryology are thus brought into mutual 
bearing on one another. 
Let us take an example in which these two principles seem to 
be illustrated. In rocks of the Silurian age there exist in t 
profusion the remarkable fossils known as graptolites. These 
consist of a series of little cups or cells arranged along the sides 
of a common tube, and the whole fossil presents so close a re- 
semblance to one of the Sertularian hydroids which inhabit the 
waters of our present seas as to justify the suspicion that the 
graptolites constitute an ancient and long since extinct group of 
the Hydroida, It is not, however, with the proper cells or hy- 
drothecze of the Sertularians that the cells of the graptolite most 
closely agree, but rather with the little receptacles which in cer- 
tain Sertularinee belonging to the family of the Plumularida we 
find associated with the hydrothece, and which are known as 
“‘ Nematophores,” a comparison of structure then shows that the 
graptolites may with considerable probability be regarded as re- 
presenting a Plumularia in which the hydrothecz had never been 
developed and in which their place had been taken by the nema- 
tophores, 
Now it can be shown that the nematophores of the living Plu- 
mularida are filled with masses of protoplasm which have the 
power of throwing out pseudopodia, or long processes of their 
substance, and that they thus resemble the Rhizopoda, whose 
soft parts consist entirely of a similar protoplasm and which 
stand among the Protozoa or lowest group of the animal kingdom. 
If we suppose the hydrotheca suppressed in a plumularian, we 
should thus nearly convert it into a colony of Rhizopoda, from 
which it would differ only in the somewhat higher mophological 
differentiation of its caenosare or common living bond by which 
the individuals of the colony are organically connected. And 
just such a colony would, under this view, a graptolite be, wait- 
ing only for the development of hydrotheca to raise it into the 
condition of a plumularian. 
Bringing now the evolution hypothesis to bear uper the ques- 
tion, it would follow that the graptolite may be viewed as an 
ancestral form of the Sertularian hydroids, a form having the 
most intimate relations with the Rhizopoda ; that hydranths and 
hydrothecze became developed in its descendants ; and that the 
rhizopodal graptolite became thus converted in the lapse of ages 
into the hydroidal Sertularian. 
This hypothesis would be strengthened if we found it agreeing 
with the phenomena of individual development. Now such 
Plumularida 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 hydrothecz, 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 development of the tribe. 
I have thus dwelt at some length on the doctrine of evolution 
because it has given a new direction to biological study and must 
powerfully influence all future researches. Evolution is the 
highest expression of the fundamental principles established by 
Mr. Darwin, and depends on the two admitted faculties of living 
beings—Aeredity, or the transmission of characters from 
parent to the offspring ; and adaptivity, or the capacity of hav- 
ing these characters more or less modified in the offspring by 
external agencies, or it may be by spontaneous tendency to 
variation, 
The hypothesis of evolution may not, it is true, be yet esta- 
blished on so sure a basis as to command instantaneous acceptance, 
and for a generalisation of such vast significance no one can be 
blamed for demanding for it a broad and indisputable foundation 
of facts. Whether, however, we do 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 3 them than any other hypothesis which has yet been pro- 
posed. 
With all our admiration, however, for the doctrine of Evolu- 
tion as one of the most fertile and comprehensive of philosophic 
hypotheses, we cannot shut our eyes to the difficulties which lie 
