be
taken into account in order to classify them scientifically; and, also,
that for this purpose the internal parts were of quite as much
importance as the external. Indeed, he perceived that they were of
greatly more importance in this respect, inasmuch as they presented so
many more points for comparison; and, in the result, he furnished an
astonishingly comprehensive, as well as an astonishingly accurate
classification of the larger groups of the animal kingdom. On the other
hand, classification of the vegetable kingdom continued pretty much as
it had been left by the book of Genesis--all plants being divided into
three groups, Herbs, Shrubs, and Trees. Nor was this primitive state of
matters improved upon till the sixteenth century, when Gesner
(1516-1565), and still more Caesalpino (1519-1603), laid the foundations
of systematic botany.

But the more that naturalists prosecuted their studies on the anatomy of
plants and animals, the more enormously complex did they find the
problem of classification become. Therefore they began by forming what
are called artificial systems, in contradistinction to natural systems.
An artificial system of classification is a system based on the more or
less arbitrary selection of some one part, or set of parts; while a
natural classification is one that is based upon a complete knowledge of
all the structures of all the organisms which are classified.

Thus, the object of classification has been that of arranging organisms
in accordance with their natural affinities, by comparing organism with
organism, for the purpose of ascertaining which of the constituent
organs are of the most invariable occurrence, and therefore of the most
typical signification. A porpoise, for instance, has a large number of
teeth, and in this feature resembles most fish, while it differs from
all mammals. But it also gives suck to its young. Now, looking to these
two features alone, should we say that a porpoise ought to be classed as
a fish or as a mammal? Assuredly as a mammal; because the number of
teeth is a very variable feature both in fish and mammals, whereas the
giving of suck is an invariable feature among mammals, and occurs
nowhere else in the animal kingdom. This, of course, is chosen as a very
simple illustration. Were all cases as obvious, there would be but
little distinction between natural and artificial systems of
classification. But it is because the lines of natural affinity are, as