30 THE ORGANIC GROWTH OF THE LIVING WORLD sec. 



granted at the same time the impossibility of sexual inter- 

 course, two species. 



The law of undulation, and the other laws 1 to 4 which I 

 have formulated, hold therefore as much for the origin or evo- 

 lution of the variety and the species as for the individual. 



One after another, like waves, the new characters appear as 

 stages of growth in the series of organisms which represent 

 the evolution of species. Or as we may conversely express it, 

 the higher species in the ontogeny of the individuals belonging 

 to them, which represents the evolution of those individuals, 

 briefly repeat the characters (stages of growth) of the lower. 

 The highest animals briefly repeat in their ontogeny the whole 

 series of their ancestors (biogenetic law) as stages of growth. 

 And indeed, I have to add, in general the more briefly and 

 the more incompletely they repeat the individual members of 

 the ancestral series, the older these are, the less important they 

 were, and the shorter the duration they had. 



Every older stage of the phyletic growth is abbreviated for 

 the benefit of the newer — a proposition which may be added 

 as a fifth to the four already formulated. 



Thus the facts established by me afford at the same time 

 a new and complete confirmation of the biogenetic law. 

 Varieties and species are therefore in reality nothing but 

 groups of forms standing at different stages of evolution, 

 that is, at different stages of phyletic growth, whether it be 

 that they outstripped their fellows or their fellows them 

 in the progress of evolution, so that connection by inter- 

 mediate forms was lost, or that separation in space favoured 

 separation in character. 



Isolation in space or separation of habitats is obviously of 

 great importance in species formation, but it is not absolutely 

 necessary. 



The same explanation evidently holds good for genera as 

 groups of species, and in general for all divisions of a natural 



