10/2 The American Naturalist. [December, 
animals and plants where the Hnes between the various forms are 
so indefinite that they are practically absent. The testimony of 
both botanists and zoologists can be quoted in this regard. Lind- 
ley, though regarding species as " created by Nature herself, and 
remaining always the same" (Intro, to Botany, p. 307, 1832), yet 
states that " No absolute limits exist, by which groups 
of plants can be circumscribed. They pass into each other by 
insensible gradations, and every group has apparently some 
species which assumes in part the structure of some other group " 
(Vegetable Kingdom, p. 30). 
Among zoologists Milne-Edwards says : " When zoology is 
only studied in systematic works it is often supposed that each 
class, each family, each genus, present to us boundaries precisely 
defined, and that there can be no uncertainty as to the place to 
be assigned, in a natural classification, to every animal the organi- 
zation of which is sufficiently known. But when we study this 
science from Nature herself we are soon convinced of the con- 
trary, and we sometimes see the transitions from one plan of 
structure to an entirely different scheme of organization take place 
by degrees so completely shaded one into the other, that it be- 
comes very difficult to trace the line of demarcation between the 
groups thus connected" {Amer. Sci. Nat., Sept., 1840, — quoted 
by Lindl., Veg. Kingd., p. 31). 
Nature recognizes but one class in her domain. That class is 
composed of individuals, and the individuals are her units. So, 
too, they are the units of man's classification, and for his own 
convenience he groups them into what he calls species ; the spe- 
cies he arranges in genera, and the genera are collected in famil- 
ies or orders. Such a classification is necessarily more or less 
arbitrary, however natural it may be considered ; and it is essen- 
tially artificial, inasmuch as no such grouping exists in Nature. 
Among the individuals there is always a greater or less amount 
of variation. Sir Morell Mackenzie tells us that the muscles that 
form the human larynx are not arranged alike in any two indi- 
viduals ; and that differences in physiognomy are probably due 
to variations in arrangements of the muscles which move the 
skin of the face {Pop. Sci. Monthly, December, 1889). Though 
