Ch. xiii, on Intermediates between Varieties and the Pure Lines of Johannsen, is of crucial importance. Intermediate and gradational forms undoubtedly appear; how can Mendelism cope with them? Professor Bateson grants that analysis is as yet incomplete and must be laborious; but he urges that in many cases the intermediate character is provably only a superficial or net result of the interaction of factors which are transmitted as units. Sometimes, e. g. t the whole group of heterozygotes forms a recognizable class which may be described as intermediate between the two pure types (blue Andalusian fowl). Or intermediates may be due to subtraction-stages of dominant factors (color of the Dutch rabbit; half-dwarf ness in peas), or to the interference of other factors (English pattern of rabbit; Painted Lady form of Sweet Pea). And finally there are intermediates due to the disturbing effects of many small causes not of genetic but presumably of environmental origin, fluctuational forms whose intermediacy is not transmissible. In view of all these possibilities, it is evidently incautious to assert that in any specific case segregation does not occur.
Ch. xiv takes up certain Miscellaneous Exceptional and Unconformable Phenomena, to wit, cases in which crosses breed true without segregation, departures from numerical expectation, irregularities of dominance, alternation of generations, maternal characters in embryos. In most instances, the author is able to suggest at any rate a possible and plausible explanation of the anomaly. Alternation of generations is, however, as he confesses, a phenomenon which at present is incapable of factorial representation.
Ch. xv briefly considers Biological Conceptions in the Light of Mendelian Discoveries. "Much that is known of chromosomes seems inconsistent with the view that they are the sole effective instruments in heredity." Variation must be regarded in the main as a phenomenon due to the addition or omission of one or more definite elements. Reversion occurs when the sum-total of the factors returns to that which it has been in some original type; reversion on crossing is thus merely the special case in which one or more missing factors are brought in by the parents of the cross-breed. As for the bearing of Mendelism on the theory of evolution, the following may be said, (i) In countless instances segregation plays a part in the constitution and maintenance of characteristics held by systematists to be diagnostic of species. De Vries* distinction of specific and varietal (nonsegregating and segregating) characters cannot be accepted. (2) There is a real difference between fluctuating variations and actual genetic variations. By the latter alone can permanent evolutionary change pf type be effected; and they are commonly, though not always, sufficiently discontinuous to merit the name Mutations. (3) There is nothing in Mendelism that runs counter to the doctrine of Natural Selection, although the scope of that principle is closely limited by the laws of variation.
The concluding chapter xvi, on the Practical Application of Mendelian Principles, should be read in connection with the Preface, which extends the discussion of what the fancier or breeder has to expect from Mendelism. Mendelian discovery, as we have seen, abolishes the old idea that time and continued selection are needed in order to make a variety breed true. Certain types are unfixable, for the simple reason that their special character is a special consequence of the meeting of dissimilar gametes. Sociologically, Mendelism suggests a mode of procedure the opposite of that favored by current eugenics; certain serious physical and mental defects, almost certainly also some morbid diatheses, and some of the forms of vice and criminality
