388 SCOTT. [Vol. V. 



species of the successive genera Palceosyops, Diplacodon, and 

 Titanotlierium, of Oreodon and Eporeodon, and many others. On 

 the other hand, it is by no means always the case that those 

 genera which exhibit the greatest amount of variability at any 

 given time, are those which in the long run have undergone the 

 most extensive " mutation " ; on the contrary, such genera 

 frequently exist for very long periods substantially unchanged, 

 but showing great variability at every stage in their history. 



These facts at least suggest the possibility that individual 

 variations are not incipient species, but that the causes of trans- 

 formation lie deeper, and act with more or less uniformity upon 

 large numbers of individuals. It may, perhaps, be the outcome 

 of future investigations, that while variations are generally due 

 to the union of changing hereditary tendencies, mutations are the 

 effect of dynamical agencies operating long in a uniform way, 

 and the results controlled by natural selection. While this may 

 be true, a great many facts must be gathered in its support, 

 before it can be regarded as more than a suggestion. 



In particular, it is difficult to see how a new character can arise 

 from the crossing of two forms, neither of which possess that 

 character. Some facts are, it is true, recorded, which seem to 

 show that new characters may thus arise, as for example in cer- 

 tain hybrid plants which exhibit features present in neither of the 

 parent species. But, for the present, such facts must be received 

 with great reserve, for the analogy of domesticated breeds shows 

 that crossing favors reversion, and in every case the supposed 

 new characters may be atavistic. At least, no conclusion can be 

 safely drawn from such facts, until it is shown that reversion is 

 excluded. To avoid this difficulty as to the production of new 

 characters, Weismann assumes that all variations are quantita- 

 tive and not qualitative. This is equivalent to saying that there 

 are no qualitative differences in the organic world. No summa- 

 tion of quantitative differences can bring about chemical changes ; 

 for example, the first appearance of muscle, cartilage, and the 

 various tissues cannot thus be accounted for, and if the principle 

 be abandoned for these cases, its importance is at once destroyed. 

 The only escape from this difficulty is the suggestion of Lloyd 

 Morgan, already quoted, that impregnation represents not a 

 mere commingling, but a combination analogous to chemical 

 union ; but for this there is no evidence, and Morgan himself is 



