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 ^g^ is only an 

 analogy. 



The efl&cient cause in the first case, the slow modifi- 

 cation of the race by tlie 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 ej^igenesis, the conclusion 

 that the ^gg which is to become a man differs in no 

 essential particular from the Qgg 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 



