38 



ORTHOGENETIC EVOLTTTION IN PIGEONS. 



in successive plumages as to completely forestall the saltation hypothesis. Not 

 only is the direction of the change hitherto discoverable, but its future course is 

 predictable. In fact, a few allied species are found in which the change has been 

 carried farther and in the direction plainly anticipated in the species we are con- 

 sidering. One species {Lophophaps leucogaster) , also a crested pigeon and confined to 

 Australia, shows this same variation carried through the whole row of long coverts 

 (text-fig. 9), and already moving down the last row of feathers in the wing. Only 

 4 of the elongated metallic spots are left in this last row, while 5 to 7 are to be seen 

 in Ocyphaps. The number falls to 3 in another species (Lophophaps plum if era). 



For questions involving the vital principles of the prevailing theories of organic 

 evolution, the usual difficulty is to find tests that are really conclusive. As the 

 main question turns on the nature of the relation between two specifically distinct 

 conditions of a character, one of which is known or conjectured to be a variation 

 or a mutation from the other, it is generally assumed, as a matter of course, that 



Text-figure 11. — Adult female crested pigeon, Ocyphaps lophotes. x 0.8. Hayashi del., L898. 

 Spots and bars of left wing. Compare with Juvenal crested (text-figure 10) and with examples 

 of Geopelia. 



we must go to two distinct but allied species for the two conditions to be com- 

 pared. Variates are collected from both sides in order to see if they can be arranged 

 in a single series running so continually from one species into the other that it will 

 be impossible to find the dividing-line. Such series can be readily formed in number- 

 less cases, but, as de Vries has so well shown, they supply no test. De Vries states : 



The overstepping of the limits occurs only in single and relatively rare individuals; the 

 great majority belong to the mid-type of their species. If therefore we do not search for 

 transitions, or if we do not seek merely to complete the series, but rather to make the 

 measurements as numerous as possible, then the curves will come to light. Precisely that 

 will come -out which we noted in the consideration of the Oenothera flowers. The well- 

 known multimodal curves studied by Bateson, Ludwig, and many other investigators will 

 appear. Every modal summit corresponds to a group of individuals belonging together— 

 a type, or an elementary species. 



The transition-forms are easily detected by their infrequency. It becomes evident at 

 once that they only apparently obliterate the limits, and that they can not possibly bring 

 into confusion the renters of greatest frequency. They prove nothing beyond the fact 

 that neighborin g curves on the same abscissa may overlap each other with their limbs. 2 



2 Die Mutationstheorie, I, p. 307. 



