IV.] FOUNDATION OF A THEORY OF HEREDITY. 205 



become visible periodically. He believes that the passage from 

 one phyletic stage to another is chiefly due to the fact that ' in 

 any ontogeny, the very last structural change upon which the 

 separation of germs depends, takes place in a higher stage, one 

 or more cell-generations later' than it occurred in a lower stage, 

 ' The last structural change itself remains the same, while the 

 series of structural changes immediately preceding it is in- 

 creased.' I believe that Nageli, being a botanist, has been too 

 greatly influenced by the phenomena of plant-life. It is cer- 

 tainly true that in plants, and especially in the higher forms, 

 the germ-cells only make their appearance, as it were, at the 

 end of ontogeny; but facts such as these do not hold in the 

 animal kingdom : at any rate they are not true in the great 

 majority of cases. In animals, as I have already mentioned 

 several times, the germ-cells are separated from the somatic 

 cells during embryonic development, sometimes even at its 

 very commencement ; and it is obvious that this latter is the 

 original, phyletically oldest, mode of formation. The facts at 

 our disposal indicate that the germ-cells only appear, for the 

 first time, after embr3^ological development, in those cases 

 where the formation of asexually produced colonies takes place, 

 either with or without alternation of generations ; or in cases 

 where alternation of generations occurs without the formation 

 of such colonies. In a colony of polypes, the germ-cells are 

 produced by the later generations, and not b}'' the founder of 

 the colony which was developed from an ^gg. This is also 

 true of the colonies of Siphonophora, and the germ-cells appear 

 to arise very late in certain instances of protracted metamor- 

 phosis (Echinodermata), but on the other hand, they arise 

 during the embryonic development of other forms (Insecta) 

 which also undergo metamorphosis. It is obvious that the 

 phyletic development of colonies or stocks must have succeeded 

 that of single individuals, and that the formation of germ-cells 

 in the latter must therefore represent the original method. Thus 

 the germ-cells originally arose at the beginning of ontogeny and 

 not at its close, when the somatic cells are formed. 



This statement is especially supported by the history of 

 certain lower plants, or at any rate chlorophyll-containing 

 organisms, and I think that these forms supply an admirable 

 illustration of my theory as to the phyletic origin of germ- 



