40 ORTHOGENETIC EVOLUTION IN PIGEONS. 



that it seems to stand in sharp contrast with changes of a specific nature, which, 

 according to de Vries, are fixed on their first appearance. 



In biometric dress, individual variations present themselves as oscillations to 

 and fro in a linear direction, while mutations appear to diverge in various directions. 

 On the one side, every advance is sooner or later lost in regression; on the other, 

 regression never appears. Variability seems to rise and fall with the food-supply, 

 while mutation is not so affected. 



Mutation appears with the suddenness and completeness of a chemical substi- 

 tution, and it can no more pass into a new specific form through flowing variation 

 than could a crystal be slowly transformed into another of a totally different nature. 

 In such respects, variability is represented as always standing at the opposite 

 pole; cumulative to a degree, but without permanency; plus or minus, but never 

 qualitative. 



The mutation theory puts the origin of species beyond the reach of investiga- 

 tion, shrouding it in the utter darkness of premutation stages. The idea of fixity 

 as an essential characteristic of species bars the way to accounting for the origin of 

 the most remarkable phenomena of the organic world, namely, adaptations. The 

 characterization of variability as linear is too superficial to be of diagnostic value. 



If fertility is a thing of all degrees, and that, too, within the limits of a single 

 species, are not the qualitative distinctions inferred therefrom likewise things of 

 degrees and variable within the same limits? If fertility may vary individually, 

 as is certain, is there any reason to doubt that its physical bases are equally variable 

 from individual to individual? But fertility varies also according to age and con- 

 ditions of food, temperature, etc. 4 How then can qualitative differentiations be 

 an infallible criterion of species? 



But discussion of such questions is somewhat aside the main purpose here. It 

 is more profitable to take the fundamental issue to the test of facts. This issue is 

 found in the denial of transitional stages in the evolution of species. If indubitable 

 transitional stages between indubitable specific characters can be found, even in 

 a single case, the mutation theory falls. To catch a character in the very act of 

 giving birth to a new character, through gradual transitional steps, to be able to 

 see the genetic continuity before it is broken for the last time, that is the kind of 

 evidence most to be desired. Such evidence can only be given in a natural series, 

 which is presented in a field completely oriented. The extremes of the series must 

 be two characters specifically distinct, but not yet completely separated. The 

 transitional stages must lie between these extremes, and lead in a definite direction 

 and continuously from the later into the older character. Such transitions will 

 gradually descend in the same direction; i.e., they will depart less and less from the 

 earlier mark as they approach and pass into it. Under such conditions chance is 

 not the law, and probability curves lose their enchantment. All the bearings are 

 given by direct inspection, and no dispute can arise as to the transitional nature 

 of the connecting stages. 



All these crucial advantages are placed in our hands in the species to be 

 examined, and they are all the more decisive in that the transitional phases are 

 to be seen in several consecutive stages of evolution in the young and adult birds. 



* The data which establish these facts are given in Vol. II. ED. 



