334 THE GERM-PLASM 



sti)nulus to reversion in these instances to interbreedins: : that 

 is to say, to the amphimixis of such germ-cells as contain a 

 certain number of ancestral determinants in their germ-plasm 

 on either side for those regions of the skin in which the nipples 

 were situated in remote ancestors. Merely in consequence of 

 the reducing division, these determmants may possibly accumu- 

 late in one germ-cell in sufficient numbers to produce the 

 character in question. The same may be said with regard to 

 many small individual marks, and such very ancient hereditary 

 parts as supernumerary nipples usually follow the same rule as 

 do individual characters ; they have to a certain extent degener- 

 ated so as to come under the same category, and for a long time 

 past have not been contained in the germ-plasm of every indi- 

 vidual ; their determinants, on the contrary, are entirely absent 

 in most cases, and are only found in a certain number of ids in 

 certain individuals. These determinants, like those of individual 

 characters, may be transmitted for several generations without 

 attaining development, and may then suddenly become manifest 

 in consequence of a favorable combination of two conjugating 

 germ-cells. The only difference between these ancestral charac- 

 ters and ordinary individual peculiarities is, that the determi- 

 nants of the latter are contained in a larger number of ids, and 

 we must therefore conclude that they become developed far 

 more frequently and regularly. 



Although in the case of the supernumerary nipples in the 

 human race we cannot definitely indicate the ancestor from which 

 they were derived, or the length of time during which they have 

 been transmitted, it is at any rate possible to do so approxi- 

 mately in the case of the superniDnerary toes of horses. For, 

 thanks to the excellent researches of Kowalewsky, followed by 

 those of Marsh, we are well acquainted with the phyletic devel- 

 opment of the horse : we now know that horses belonging to 

 the genera Mesohippus, MioJiippiis, and Protohippus or Hip- 

 parion, which possessed two smaller lateral toes in addition to 

 the large median ones, existed in the middle Tertiary period. 

 When horses are occasionally born at the present day in which 

 one or two such accessory toes are present on two or even 

 on all four feet, we are perfectly right in considering the devel- 

 opment of these toes to be due to reversion to an ancestor of 

 the Miocene period. We must therefore assume that in certain 

 series of generations of the existing horse, some idants have 



