SPECIES ONLY RELATIVELY STABLE. 



103 



ing that so-called good species, in the sense of the sys- 

 tematists, have no existence, human intellect, in the en- 

 deavour to obtain a general view, would be compelled to 

 denominate the forms, unless all scientific treatment was 

 to be rendered impracticable. But the retention of spe- 

 cies is moreover 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 tak- 

 ing place by stages in organisms subject to metamor- 

 phosis, 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 individuals. 

 These remain persistent as long as they exist under the 

 same external conditions. How far time in itself afifects 

 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 abso- 

 lutely mutable, and only relatively stable, we will term 

 it, with Haeckel, " the sum of all cycles of reproduction 

 which, under similar conditions of existence, exhibit sim- 

 ilar forms." 



