24 Darwin, and after Darwin. 
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 per- 
ceived 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 Cesalpino (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 contra- 
distinction 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, 
