842 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



acters" as they appear in the body, and the demon- 

 strated separableness of their "factors," "determiners," 

 "genes "in the germ, gave rise afresh to the tlieoretic 

 assumption of discontinuity of origin of all characters 

 in the germ. In 1912 (1912.372) Osborn established 

 the fact that this assumption is a non sequitur. 



Mendelian research on the mammals has been con- 

 fined mainly to color characters in certain species of 

 rodents, chiefly mice and guinea pigs, although it has 

 been extended also to horses and cattle, which were 

 studied less by experiment than from the records of 

 stud books. The first striliing general result is the 

 principle of antithesis in the heritage of characters 

 that mutually exclude each other, as originally typi- 

 fied by the antithesis of Mendel's "tallness" and 

 "shortness," "smooth coat" and "wrinkled coat" in 

 peas. The second MendeUan principle is that when 

 these antithetic characters meet in the germ cells the 

 dominant character becomes visible or patent in the 

 offspring, whereas the recessive character remains 

 latent in the germ, perhaps to reappear in later crosses 

 in the Mendehan ratios. 



Similar dominance of head proportion appears in the 

 first cross between the ass ( c? ) and horse ( 9 ) as exem- 

 plified in the mule, which exhibits the dominant 

 dolichocephaly of the horse and at the same time the 

 dominant molar-tooth pattern of the ass. Other pro- 

 portional characters in this hybrid are known to be 

 imperfectly "dominant," or "blending." In such cases 

 inheritance is blending and non-Mendelian, for there 

 is no evidence of size character segregation in the 

 germ other than the increased variability of the second 

 hybrid generation. 



TRUTH AND EEEOK IN JOHANNSEN's PURE LINE 

 SALTATION PRINCIPLE 



The "pure line" is another biologic principle from 

 which both true and false conclusions have been 

 drawn. This principle is based upon breeding expe- 

 riments on pure strains of garden beans which tend 

 to show that certain kinds of characters arise only 

 discontinuously by saltation. Johannsen (1911.1, pp. 

 129-159), through experiments on successive genera- 

 tions of self-fertUizing plants (the garden bean), 

 reached the illogical opinion as to all plants and ani- 

 mals that the apparent continuity of visible, somatic 

 form is delusive, and that in the origin of "mutations" 

 and "species" there is invariably a germinal saltation 

 or discontinuity. His opinions may be summarized 

 and paraphrased as follows: 



The developing organism which the paleontologist or the 

 zoologist observes may be called the phenotj-pe, or visible type. 

 Vertebrate paleontologists and zoologists comparing huge col- 

 lections in museums have erected in phylogenetic speculation 

 a science of phenotypes which is not of value in a study of 

 germinal evolution (genetics) because the description of pheno- 

 types is inadequate as the starting point for genetic investiga- 

 tion. The adaptation of phenotypes through the direct influence 



of environment [Buffon's factor] or of use and disuse [Lamarck's 

 factor] is not of genetic [hereditary] importance. Ontogeny is 

 an expression of the heredity germ, but the heredity germ is 

 not affected by ontogenesis. The conception of evolution by 

 continuous transitions from one type to another has imposed 

 itself upon zoologists, botanists [and paleontologists] who are 

 examining chiefly shifting visible forms in very fine gradations. 

 There may be such a continuity in visible form but not in the 

 hereditj' germ from which they spring. All degrees of con- 

 tinuity between phenotypes may be found, but real germinal 

 transitions must be distinguished from the transitions which 

 we observe in visible museum specimens. 



The delusive nature of phenotype continuity is shown by the 

 examination of pure lines of plants. A typical pure line is 

 composed of the descendants of one pure strain of an organism 

 exclusively propagated by self-fertilization, as in the case of the 

 garden bean, which demonstrates the stability of germinal con- 

 stitution in successive generations where undisturbed by cross 

 breeding with other strains. 



Johannsen proposes the term gene to designate the germinal 

 "factors," "determiners" (of authors), of various visible char- 

 acters. The sum total of all the genes in the fertilized germ 

 cell he terms the genotype. A group of similar genotypes — that 

 is, of pure-strain individuals — he terms the biotype. The 

 genotype, the sum total of the genes, can be examined only 

 by the qualities and reactions of the phenotypes under experi- 

 ment. Such examination shows that within pure lines — if no 

 mutations (De Vries) or other disturbances have been at work — 

 there are no genotypioal (that is, germinal) differences in the 

 characters under examination. For example, the mutations of 

 De Vries observed in nature have shown themselves as consid- 

 erable discontinuous saltations. 



The fallacy of Johannsen's argument lies in the 

 dictum "ab uno disce omnes." He may be entirely 

 right as to the origin of certain germinal characters 

 and entirely wrong as to the origin of others. Never- 

 theless, he concludes from his observations on the 

 bean that the essential point in all evolution is the 

 sudden alteration, loss or gain, of the genes, the 

 germinal constituents, of the genotype. In his opin- 

 ion all evidence as to "mutations of De Vries" points 

 to the discontinuity of the changes in question. In 

 the theory of origin of allometrons, the crucial point 

 in the application of Johannsen's pure-line hypothesis 

 would be the assumed stability of the genes, factors, or 

 determiners, as distinguished from their assumed 

 fluctuation. 



EXPERIMENTS IN THE ARTIFICIAL SELECTION OF 

 VARIATIONS OF PROPORTION 



Against such hypotheses of the stability of all 

 germinal determiners of proportional, quantitative, 

 intensive characters may be set the evidence adduced 

 by many experimentalists as to the heritage of quan- 

 titative variation. As opposed to Johannsen, Castle 

 (1916.1) observes that in his experiments every 

 "unit character" — that is, every germinal deter- 

 miner — is subject to quantitative variation — that is, 

 its visible expression in the body varies — and it is 

 clear that these variations have a germinal basis, 

 because they are inherited. By selection, plus or 

 minus, through a series of generations we can intensify 



