404 
that a dictionary has in literature. The mean annual 
temperature of a given station, and the occurrence of a 
certain plant in a certain locality, are facts of kinds that 
must enter extensively into the relationships with which 
geography deals; but these facts, standing alone, are want- 
ing in the essential quality of mature geographical science. 
Not only so; many facts of these kinds may, when treated 
in other relations, enter into other sciences; for it is not so 
much the thing that is studied as the relation in which it is 
studied that determines the science to which it belongs. 
There can be no just complaint of narrowness in a science 
that has charge of all the relations among the elements of 
terrestrial environment and the items of organic response. 
Indeed, the criticism usually made upon the subject thus 
defined is, as has already been pointed out, that it is too 
broad, too vaguely limited and too much concerned with all 
sorts of things to have sufficient unity and coherence for a 
real science. Some persons, indeed, object that geography 
has no right to existence as a separate science; that it is 
chiefly a compound of parts of other sciences; but if it be 
defined as concerned with the relationships that have been 
just specified, these objections have little force. It is true, 
indeed, that the things with which geography must deal are 
dealt with in other sciences as well, but this is also the case 
with astronomy, physics, chemistry, geology, botany, 
zoology, history, economics, and other sciences. There is no 
subject of study the facts of which are independent of all 
other subjects; not only are the same things studied under 
different sciences, but every science employs some of the 
methods and results of other sciences. The individuality of 
a science depends not on its having to do with things that 
are cared for by no other science, or on its employing 
methods that are used in no other science, but on its study- 
ing these things and employing these methods in order to 
gain its own well-defined object. Chemistry, for example. 
is concerned with the study of material substances in re- 
lation to their constitution, but it constantly and most 
properly employs physical and mathematical methods in 
reaching its ends. Botanists and zoologists are much 
interested in the chemical composition and physical action 
of plants and animals, because the facts of composition and 
action enter so largely into the understanding of plants and 
animals considered as living beings. Overlappings of the 
kind thus indicated are common enough, and geography, as 
well as other sciences, exhibits them in abundance. It may 
be that geography has a greater amount of overlapping than 
any other science; but no valid objection to its content can 
be made on that ground ; the maximum of overlapping must 
occur in one science or another—there can be no discredit 
to the science on that account. Geography has to do with 
rocks the origin of which is studied in geology; with the 
currents of the atmosphere, the processes of which exemplify 
general laws that are studied in physics; with plants and 
animals, the forms and manner of growth of which are the 
first care of the botanist and zoologist; and with man, 
whose actions recorded in order of time occupy the historian ; 
but the particular point of view from which the geographer 
studies all these things makes them as much his own 
property as they are the property of anyone else. 
Some UNSOLVED PROBLEMS OF ORGANIC ADAPTATION. 
The recent impulse which has come to biologic progress 
by experimental methods, and the remarkable results which 
have been attained thereby, may without exaggeration be 
said to have raised anew many an earlier doubt as well as 
brought to light problems apparently beyond the scope of 
the older explanations. It may not, therefore, be an ex- 
travagant assumption to announce the entire question of 
organic adaptations as open for reconsideration, in the light 
of which no apology will be necessary for directing atten- 
tion to certain phases of the subject upon the present 
occasion. 
Among the many problems which recent investigations 
and conclusions have brought into better perspective as well 
as sharper definition, and which might profitably be dis- 
cussed, the limits of a single address preclude any very wide 
range of review. I have, therefore, chosen to restrict my 
discussion chiefly to problems of coloration among lower 
1 Abridged from an address delivered before the Section of Zoology of the 
American Association by Prof. C. W. Hargitt. 
NO. 1791, VOL. 69| 
NATURE 
[ FEBRUARY 25, 1904 
invertebrates, including incidental references to correlated 
subjects, and the probable limitations of colour as a factor 
in organic adaptation. 
As is perfectly well known, colour in nature is due to one 
of two causes, or to a combination of both, namely, (1) 
what has been termed optical or structural conditions, such 
as diffraction, interference or unequal reflection of light, 
examples of which are familiar in the splendid hues of the 
rainbow, the iridescent sheen and metallic colours of the 
feathers of many birds, wings of insects, &c. (2) What are 
known as pigmentary colours, due to certain material sub- 
stances lodged within the tissues of animals or plants which 
have the property of absorbing certain elements of light 
and of reflecting others, and thereby producing the sensa- 
tion of colour. While the two are physically quite distinct 
it is not unusual to find them associated in producing some 
of the most exquisite colour effects of which we have know- 
ledge. In a general way one may usually distinguish 
between these two sorts of colour by noting that those 
which are purely optical in their character produce a con- 
stantly changing impression as the relative position of 
object or observer may happen to vary with reference to the 
angle and direction of light; while, upon the other hand, 
colours which are due to pigments show this property very 
slightly or not at all, and that, moreover, pigment colours 
are usually more or less soluble in various reagents, such as 
alcohol, ether, acids, alkalies, &c., and that they often fade 
rapidly under the influence of strong light or in its absence, 
or upon the death of the organism. 
The work of Krukenburg, MacMun, Macallum, M’Ken- 
dric, Hopkins, Urech, Eisig, Cunningham, and a host of 
others, comprising a mass of literature of enormous pro- 
portions, will be available to those interested, and may 
afford some faint conception of the magnitude and import- 
ance of the field to be explored, as well as an introduction 
to that already made available. And while as a result of 
this activity many and various organic pigments have been 
isolated and their composition in part or entirely made 
known, it must be recognised that the task of the chemical 
analysis of any such highly complex compounds as most of 
these are known to be is attended with extreme difficulty 
and no small measure of uncertainty. Still, it has been 
possible fairly to distinguish several classes of such pig- 
ments, differentiated physiologically as follows :— 
(1) Those directly serviceable in the vital processes of the 
organism. Under this head may be classed such pigments 
as haemoglobin, chlorophyll, zoonerythrin, chlorocruorin, and 
perhaps others less known. It need not be emphasised that 
by far the most important of these are the two first named. 
The others, found chiefly among the lower invertebrates, are 
believed to serve a function similar to the first. 
(2) Waste products. Among these the several biliary pro- 
ducts are too well known to call for special note. Guanin 
is a pigment of common occurrence in the skin of certain 
fishes, and is associated with the coloration of the species- 
Similarly certain colouring matters have been found in the 
pigments of many Lepidoptera, known as lepidotic acid, a 
substance closely allied to uric acid, and undoubtedly of the 
nature of a waste product. 
(3) Reserve products. Of these there are several series, 
one of which, known as lipochrome pigments, is associated 
with the metabolism involved in the formation of fats and 
oils. Perhaps of similar character are such pigments as 
carmine, or rather cochineal, melanin, &c. It may be some- 
what doubtful whether these pigments do not rather belong 
to the previous class, where should probably be listed such 
products as hematoxylin, indigo, &c., all of which have been 
claimed as resultants of destructive metabolism in process of 
being eliminated from the physiologically active tissues of 
the body of the organism. Of similar character is probably 
tannic acid, a substance well known among plant products 
and involved in the formation of many of the brownish and 
rusty colours of autumn foliage, particularly of the oaks. 
and allied trees, as are the lipochromes in the formation of 
the reds and yellows which form so conspicuous a feature 
among autumn colours. 
While the association of these and other pigmentary 
matters has long been known in connection with both 
animal and plant growth, and while the conception of their 
more or less intimate relation to the active metabolism of 
the various tissues is not new, comparatively little has been 
