6 ORTHOGENETIC EVOLUTION IN PIGEONS. 



It is true that a great amount of work on Mendelian heredity seems strongly 

 to support the unit-character hypothesis, and that cytology offers some further 

 support. Nevertheless, I have to confess to wholesale skepticism. The germ, as I 

 believe and have long maintained, stands for an organized whole. It is a unit organ- 

 ism, not an organism of units; all the features that arise in course of development are 

 within the sphere of the individual unity and integral parts of it, and whatever 

 specificity they possess is completely determined and not of independent origin. 



The strongest suggestion of unit-characters is found in the Mendelian phenome- 

 non known as segregation. I do not underestimate the importance of this striking 

 behavior of so-called alternative unit-characters. I am familiar with them, and 

 deeply interested; but I am unable to see in them the sum total of all we know about 

 heredity. What I have said in regard to unit-characters applies to the Mendelian 

 doctrine. Mendelism, like mutation, neglects the natural history of the characters 

 it experiments with and is not primarily concerned to know how characters have 

 originated and multiplied. 



It seems to me a great error for the mutationist and the Mendelian to construe 

 characters as disconnected entities rather than as modifications slowly and gradually 

 evolved, in genetic continuity from stage to stage, as we see so well illustrated in 

 normal development. De Vries attaches considerable weight to the fact that 

 he has actually seen mutations and knows their pedigree for one or a few genera- 

 tions. Since the publication of his remarkable work, other investigators have 

 reported numerous mutations, and I must join this group to the extent of admitting 

 that I have witnessed phenomena that a mutationist would probably claim as 

 confirmations of his theory. Some of these phenomena require a very detailed and 

 specific treatment; but let us here define the nature of the field we are to enter and 

 make clear the way of approach. 



The ability to interpret evolutional phenomena, or even fully to understand a 

 given interpretation, implies not only concentrated attention but also a prepared 

 state of mind. I wish, therefore, first of all, briefly to discuss a query that may 

 obtrude itself at the outset. The simple lifeless color-mark of a feather may, at 

 first thought, appear to be artificial, too extremely variable, to deserve serious 

 attention. But is not variation the foundation of all evolution? and is it not a 

 great advantage to have our subject-matter where it can be easily seen? If the 

 very exuberance of these colors and color-patterns be somewhat appalling on first 

 approach, it costs but little effort to see that it all counts in favor of the investigator. 

 The greater the variability, the closer, in general, will be the connections between 

 stages, and the easier it will be to catch the trend of derivation, and to discover the 

 common points of departure for whole groups of related color-patterns, and, pos- 

 sibly, to reduce these points to a single point of departure for the whole bird king- 

 dom. Such a vista once opened would orient the whole field, disclose the direction 

 or directions of evolution, provide the investigator with a key to the natural order 

 of sequence in color-patterns, enable him to detect and to demonstrate orthogenetic 

 evolution, if such there be, and to discriminate nicely between this and the results 

 to be ascribed to natural selection and other intervening factors. From such a 

 vantage-ground, juvenile stages of color-patterns would become luminous as re- 

 capitulations, in the sense of the biogenetic law, and not stand as isolated prodigies 

 of mutation. 



