MONISTIC EVOLUTION I27 



observation and study, though some of its processes 

 are mysterious and yet involved in doubt and uncer 

 tainty. Phylogenesis is the supposed development of 

 a species in the course of geological time, and by the 

 intervention of long series of species, each in its time 

 distinct, and composed of individuals each going 

 regularly through a genetic circle of its own. 



The latter is a process not open to observation 

 within the time at our command ; purely hypothe 

 tical, therefore, and of which the possibility remains 

 to be proved, while the causes on which it must 

 depend are necessarily altogether different from those 

 at work in ontogenesis ; and the conditions of a long 

 series of different kinds of animals, each perfect in its 

 kind, are equally dissimilar from those of an animal 

 passing through the regular stages from infancy to 

 maturity. The similarity in some important respects 

 of ontogenesis to phylogenesis was inevitable, pro 

 vided that animals were to be of different grades of 

 complexity, since the development of the individual 

 must necessarily be from a more simple to a more 

 complex condition. On any hypothesis, the parallel 

 ism between embryological facts and the history of 

 animals in geological time affords many interesting 

 and important coincidences. Yet it is perfectly 

 obvious that the causes and conditions of these two 

 successions cannot have been the same. Further, 

 when we consider that the embryo cell which deve 

 lops into one animal must necessarily be originally 



