SPECIES ONLY RELATIVELY STABLE, 103 
that so-called good species, in the sense of the systematists, 
have no existence, human intellect, in the endeavour to 
obtain a general view, would be compelled to denominate 
the forms, unless all scientific treatment was to be ren- 
dered impracticable. But the retention of species is more- 
over scientifically justifiable and necessary, if only the 
determining impulses be taken into account, and the 
definition reduced to harmony with reality. Species 
is not constituted merely of analogous individuals, for 
even the sexes, in the course of development, and without 
transformation, diverge considerably from one another. 
But if we remember the transmutation of shape taking 
place by stages in organisms subject to metamorphosis, 
and the regular sequence of forms alternating with one 
another in heterogenesis, we shall be obliged to speak, 
not of individuals, but of the cycles of reproduction 
which comprise the various phases and series of indi- 
viduals. These remain persistent as long as they exist 
under the same external conditions. How far time in 
itself affects existence and decay is unknown. At any 
rate, time, as well as the external conditions of time, is 
a factor in the mutation of species. While we regard 
species as absolutely mutable, and only relatively stable, 
we will term it, with Haeckel, “the sum of all cycles of 
reproduction which, under similar conditions of exist- 
ence, exhibit similar forms,” 
