76 Heredity. 



are, to a certain extent, open to observation and study, 

 gives us ground for believing that we may hope to dis- 

 cover what it is in the structure of the egg, which ren- 

 ders these properties possible. There have been many 

 attempts to do this, but it is impossible to accept any 

 hypothesis which has ever been advanced. The evo- 

 lution hypothesis, as advocated by Bonnet and Haller, 

 is directly contradicted by the discoveries in the modern 

 science of embryology, and it is accordingly now re- 

 garded as having only an historical interest, but the 

 modern epigenesis hypothesis is no more satisfactory, 

 for the resemblance between the evolution of a species 

 from an unicellular ancestor and the development of an 

 individual animal from an unicellular egg is only an 

 analogy. 



The efficient cause in the first case, the slow modifi- 

 cation of the race by the natural selection of the most 

 favorable variations, is absent in the second case, and 

 there is nothing whatever to take its place. The paral- 

 lelism between embryology, or the ontogenetic develop- 

 ment of the individual, and phylogeny, or the evolution 

 of the race, is one of the most remarkable and instruct- 

 ive generalizations of modern science, and the very ex- 

 istence of the parallelism gives us every reason to hope 

 that an explanation of heredity or of ontogenetic devel- 

 opment may be discovered: but to point out the paral- 

 lelism is, in no sense whatever, to explain heredity. 



If the conclusion be true which is accepted by most 

 of the modern advocates of epigenesis, the conclusion 

 that the egg which is to become a man differs in no 

 essential particular from the egg which is to become a 

 starfish, heredity is an insoluble mystery, for we neither 

 possess nor have any grounds for believing that we ever 

 shall possess any knowledge of forces competent to pro- 



