ii8 Darwin, and after Darwin. 



I will now pass on to consider the embryogeny of 

 the Metazoa, beginning at its earliest stage in the 

 fertilization of the ovum. And here it is that the 

 constructive argument in favour of evolution which 

 is derived from embryology may be said properly to 

 commence. For it is surely in itself a most suggestive 

 fact that all the Metazoa begin their life in the same 

 way, or under the same form and conditions. Otnne 

 vivum ex ovo. This is a formula which has now been 

 found to apply throughout the whole range of the 

 multicellular organisms. And seeing, as we have just 

 seen, that the ovum is everywhere a single cell, the 

 formula amounts to saying that, physiologically 

 speaking, every Metazoon begins its life as a Pro- 

 tozoon, and every Metaphyton as a Protophyton^ 



Now, if the theory of evolution is true, what should 

 we expect to happen when these germ-cells are fer- 

 tilized, and so enter upon their severally distinct 

 processes of development? Assuredly we should 

 expect to find that the higher organisms pass through 

 the same phases of development as the lower or- 

 ganisms, up to the time when their higher characters 

 begin to become apparent. If in the life-history of 

 species these higher characters were gained by gradual 

 improvement upon lower characters, and if the de- 

 velopment of the higher individual is now a general 

 recapitulation of that of its ancestral species, in studying 

 this recapitulation we should expect to find the higher 

 organism successively unfolding its higher characters, 

 from the lower ones through which its ancestral species 

 had previously passed. And this is just what we do 



^ Even when propagated by budding, a multicellular organism has 

 been ultimately derived from a germ-cell. 



