3zz 



THE QUARTERLY REVIEW OF BIOLOGY 



typical development. It is therefore not 

 surprising that the present writer tried to 

 enlarge the conception of the gene and its 

 action, which he regards as proven by 

 experiment, into a general theory of 

 heredity ('17, zob, 'z 7 a). The general 

 basis of this is the conception of differ- 

 ential and balanced (abgestimmte) reaction 

 velocities. We have seen already that 

 Plunkett's work led him to accept this 

 conclusion. The present writer has fur- 

 ther tried ('z 7 a) to work out in detail 

 this general conception and to apply it to 

 the details of development as analysed in 

 experimental embryology. Studying the 

 facts in the light of the basic conception 

 he arrived at a consistent and simple 

 theory of heredity, which brings together 

 the facts of genetics as well as of experi- 

 mental and descriptive embryology. This 

 theory, which certainly goes beyond the 

 arbitrary limits set by some to "sound" 

 genetics and to "sound" physiology, 

 will not be reviewed here. But it might 

 at least be compared briefly with the only 

 other attack on the problem, Bridges' 

 conception, not yet elaborated, of the 

 genie balance. This will be of special 

 interest because the type of facts (inter- 

 sexuality) and the type of explanation 

 (balance of sex-genes) from which Bridges 

 and the present writer started was iden- 

 tical. Bridges' idea will best be presented 

 in his own words ('zz) : 



"Each gene is essentially a factory which is 

 manufacturing a characteristic set of chemical prod- 

 ucts that are delivered to the common cytoplasm, 



and that produce development through interaction 

 with each other and with materials from outside. 

 But since the chemicals produced by the different 

 genes are different, some genes will have much effect 

 upon one character and little effect upon another, 

 so that a relatively small proportion of the genes will 

 be actively concerned in producing any given char- 

 acter. Some of these genes tend to make the char- 

 acter more pronounced and others tend to make it less 

 pronounced, so that the grade of development actually 

 realized by each particular character will be deter- 

 mined by the equilibrium between its modifying 

 genes . The forms into which a given character can be 

 modified are in general quite diverse, but for the sake 

 of simplicity we may call them all + or - modifica- 

 tions. If the effectiveness of a given + or — modifier 

 is changed by mutation, the grade of the character 

 will shift correspondingly." 



This statement is rather general and it 

 will hardly be possible to account for 

 orderly development without substituting 

 some concrete conception for the gener- 

 alities. When we read that "the grade 

 of development actually realized by each 

 particular character will be determined by 

 the equilibrium between its modifying 

 genes" we see at once, I believe, that 

 this statement is endowed with a concrete 

 meaning only if we make the step from 

 the balanced genes to the balance of the 

 action of the genes. Such a balance 

 however, working in development, can 

 hardly be conceived otherwise than in the 

 form of balanced rates, of a harmony of 

 timed chains of reactions, playing to- 

 gether in an orderly, balanced, properly 

 tuned way. In other words, the concep- 

 tion of genie balance, if interpreted in 

 concrete terms, leads to our theory of 

 balanced action of the gene. 



LIST OF LITERATURE 



Baltzer, F. 1914. Die Bestimmung des Geschlechts 

 nebst einer Analyse des Geschlechtsdimorphismus 

 bei Bonellia. Mitt. Zool. Stat. Neapel, 19ZZ. 



. 1915. Untersuchungen uber die Entwick- 



lung und Geschlechtsbestimmung der Bonellia. 

 Publ. Staz. zool. Napoli, 6. 



Baur, E. 1914. Untersuchungen iiber das Wesen, 



die Entstehung und Vererbung von Rassenun- 

 terschieden bei Antirrhinum majus. Bibl. Genet., 

 4- 

 Bridges, C. B. 1917. Deficiency. Genetics, z. 



. 1911. The origin of variations in sexual 

 and sexlimited characters. Amer. Nat., 56. 



