SPECIES ONLY RELATIVELY STABLE. IO3 



that so-called good species, in the sense of the sy.-tematists, 

 have no existence, human intellect, in the endeavour to 

 obtain a general view, would be compelled to denominate 

 (he 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." 



