THE PHENOMENA OF REVERSION 335 



been retained in which ancestral determinants of the fore and 

 hind-feet are present, but that these are in the minority, and a 

 large number of them can only accumulate in one germ-cell 

 when a particularly favourable reducing division occurs. This, 

 however, would not in itself be sufficient to produce the character 

 in question : chance must also play a part, in order that such an 

 egg-cell, containing an abundance of ancestral determinants, 

 may be fertilised by a spermatozoon in which a certain number 

 of these determinants are also contained. Then, and then only, 

 will there be a likelihood that the entire number of the latter 

 will be sufficiently large to preponderate over the modern deter- 

 minants of the foot in the process of ontogeny. 



Cases of reversion of this kind occur very rarely. Marsh has, 

 however, brought forward a small series of such instances, the 

 oldest of which relates to a horse belonging to Julius Caesar, and 

 the most recent was observed by him in a living animal.* 



5. Preliminary Suvuiiary of Sections 1-5. 



All the phenomena of reversion which have so far been 

 considered can be explained on the supposition that every 

 germ-plasm is composed of a large number of equivalent units 

 or ids, each of which possesses all the determinants required for 

 the development of an organism ; every character is therefore 

 produced by the co-operation of many determinants of the 

 same region (homologous determinants). The transformation 

 of a species or race into a new one, moreover, never depends 

 primarily on a simultaneous modification of all the ids and 

 determinants : when the modification begins to take place, even 

 entire groups of ids (idants) may remain unmodified, and this 

 will subsequently be the case as regards a minority of ids, and 

 still later at least with regard to certain of the determinants in 

 individual ids. The characteristic form of every individual cell 

 which takes part in the process of ontogeny is the result of the 

 struggle of the ids which occurs in this cell ; and as some 

 amongst the mass of determinants whicli constitute every id in 

 the germ-plasm may become modified and others not, and the 

 proportion of modified to unmodified determinants may vary 

 from id to id, we can understand that in crosses between species 



* O. C. Marsh, ' Recent Polydactyle Horses,' American Journal of 

 Science, Vol. xliii., .Apri! 1892. 



