More copious details concerning mutations will be found in the publications cited. The question to be considered here is how far the claim is justified that the facts of this kind hitherto discovered afford an explanation of the process of evolution. It seems probable that mutations are of different kinds, as exemplified in _Oenothera_ by _gigas_ and _rubricalyx_ respectively, the former producing only sterile hybrids, the latter behaving exactly like a Mendelian unit. There can be little doubt that, as Bateson states, numerous forms recognised as species or varieties in nature differ in the same way as the races or breeds of cultivated organisms which differ by factors independently inherited. There are facts, however, which prove that all species are not sterile _inter se_, and that their characters when they are hybridised do not always segregate in Mendelian fashion. John C. Phillips, [Footnote: _Journ. Exper. Zool._, vol. xviii., 1915.] for example, crossed three wild species of duck, _Anas boscas_ (the Mallard) with _Dafila acuta_ (the Pintail) and with _Anas tristis_. In the former cross he states that except for one or two characters there seemed to be no more tendency to variation in the _F2_ generation than in the _F1_. An _F1_ Pintail-Mallard [female] was mated with a wild Pintail [male]. According to Mendelian expectation the offspring of this mating should have been half Pintail and half Pintail-Mallard hybrids, but Phillips states that on casual inspection the plumage of all the males appeared pure Pintail although the shape was distinctly Mallard-like. The statement is, however, open to criticism. The question is, what were the unit characters in the parent species? If the unit characters were very small and numerous, an individual in which all the characters of the Pintail existed together among the offspring of the hybrid mated with pure Pintail would be rare in proportion to the individuals presenting other combinations. Of the _F2_'s obtained from crossing _Anas tristis_ [male] with _Anas boscas_ [female] Phillips obtained 23 females and 16 males. The females were all alike and similar to _F1_ females. Of the males one was a variate specially marked, about half-way between the _F1_ type and the Mallard parent. This, according to Phillips, was a segregate. The rest showed a range of variation but no distinct segregation. It is somewhat surprising that Mendelian experts, who seem to believe that species are distinguished by Mendelian characters, have not made systematic experiments on the crossing of species in order to prove or disprove their belief.