326 



SCIENCE. 



[Vol. XX. No. 514 



relative numbers of the sexes among these hybrids, or it is due to 

 the fact that females assume abnormal secondary sexual charac- 

 ters less readily than males. Until it be proven that hybridism 

 does cause a disparity in the numbers of the sexes we may safely 

 accept the latter explanation. 



3. The preponderance of hybrids showing typical coloration of 

 cafer combined with the red nuchal crescent of auratiis con- 

 trasted with the scarcity of those showing the yellow and black 

 characters of auratus shows a predisposition to acquire red in 

 that part quite in accordance with the general law of coloration 

 in the Picidte, and may be considered a reversion to the charac- 

 ters of some common tropical ancestor from which the two spe- 

 cies have originally been derived. 



4. This tendency to assume red in preference to yellow or 

 black colors fully accords with the southern dispersion of hybrids 

 into cafer territory as contrasted with their non-dispersion into 

 auratus territory. 



5. The absence of records of pure auratus and cafer birds 

 pairing together, and the abundance and evident fertility of the 

 hybrids in some regions indicates the majority of hybrids are 

 mongrels, i. e., the offspring of hybrid parent or parents as dis- 

 tinguished from those generated by distinct species. 



6. It has been determined ' that mongrels show a stronger ten- 

 dency to revert to the characters of either parent than do hydrids. 

 In caferauratus this would result in the final elimination of the 

 hydrid element among these species. But we have seen that the 

 tendency is toward an increase of this element. 



A probable explanation of this may be found in the non appear- 

 ance of mixed characters in female hybrids by which pure-bred 

 males are readily induced to pair with them and renew the ten- 

 dency to variability. 



Among the most significant queries which spontaneously arise 

 in the mind regarding the case in hand, we may consider the 

 following with possible profit: first, how did it happen ? second, 

 when did it happen? third, what will be the result? and fourth, 

 what part has hybridization in the evolution or extinction of 

 species ? 



Bearing upon the first question the elTect of migration is of 

 special import. In general, flickers are very hardy birds, able to 

 resist the severest weather in sheltered localities as far north as 

 the forty-Hfth parallel. Over the country south of this the mi- 

 gration is less a southerly movement than a descent from the 

 mountains into the valleys and a retreat to the densely wooded 

 regions of the sea-coast. That the same conditions prevail on the 

 Pacific coast I am assured by Mr. Fannin. 



In the vast central territories of the continent north of the 

 Rocky Mountains the southerly migration is more decided and 

 far-reaching. A look at the map will show that the Rockies, 

 after extending nearly due north through the United States from 

 the headwaters of the Rio Grande to the northern boundary, sud- 

 denly contract from their easterly amplification in Montana and 

 incline far to the north west through British Columbia. South 

 of the boundary along the eastern and western slopes of this vast 

 landmark the migrating hosts of interior flickers of each species 

 would respectively pass without much admixture. But in the 

 headwaters of the Missouri region this movement becomes more 

 complicated owing to the westerly configuration of the mountain 

 system and the corresponding westward extension of the habitat 

 of auratus north of it toward southern Alaska. Here the migra- 

 tory movements of auratus first assume the character of an 

 actual invasion of the habitat of eafer, and as we go further west 

 the southerly migration of auratus from Alaska is directed by 

 physiographic and climatic conditions to the shores of the Pacific, 

 along which, from Sitka to California, resides the darker race of 

 cafer known as Colaptes cafer saturatior. 



It is here that condition^ exist more favorable to hybridism 

 between cafer and auratus than anywhere else along the frontier 

 of their common distribution, and it appears extremely probable 

 that the north-west coast of British Columbia was the first wit- 

 ness to their notorious alliance. 



Viewed thus, the history of the distribution and evolution of 

 these species over North America becomes of special interest. 

 ■ Origin of Species, p. 261. 



Starting with a common ancestry in the tropics and diverging 

 northward over the great eastern and western mountain systems 

 of the continent, they became differentiated in accord with the 

 dissimilarity of their environments. 



Readily adaptable to extremes of climate, both forms rapidly 

 extended their northerly range into the border lands of the glacial 

 epooh, auratus following its receding pathway along the Appala- 

 chian system into the Canadian lowlands and across British 

 America in the westerly direction of the Boreal life zone, while 

 cafer, spreading over the table lands of Mexico and across the 

 Mexican boundary, reached the west base of the Rocky Mountains, 

 between which and the Pacific it continued to extend until the 

 changed climatic conditions of the North Pacific coast -were en- 

 countered. From this point, having assumed the darker coloring 

 of Cleoptes cafer saturatior it rapidly extended, under more fa- 

 vored circumstances, until it met the southward migration of 

 Alaskan auratus with the result already described. 



Bearing upon the second question, that of chronology, we 

 have first pretty sure evidence of a recent extension of the habi- 

 tat of auratus into more southern territory, where it had not for- 

 merly been recorded. Coupled with this is discovered a growing 

 abundance of hybrids in the same region, indicating an aggressive 

 movement of auratus into new territory. 



From the rate of this movement and the breadth of common 

 ground over which these hybrids breed it would appear to be of 

 comparatively recent inception, possibly within the last few hun- 

 dred years. 



Owing to the scarcity of intermediate birds in what is consid- 

 ered the rightful habitat of auratus, the transmigration has ap- 

 parently come from the north and east, over neutral territory, until 

 the habitats of auratiis and cafer adjoined along a line consider- 

 ably east of the base of the Rocky Mountains, following their ex- 

 tension into British America and crossing them about latitude 

 33^ to the Pacific coast. At this phase of their history the two 

 species were probably unadulterated, the mountains continuing in 

 a modified degree to act as a natural barrier to their further ex- 

 tension. These conditions having now made possible the ac- 

 quaintance of the species, it is for us to examine whether there 

 were any characters shared by them in common which w ould 

 predispo.=ie the birdsito more intimate relations and account for 

 their apparently anomalous conduct. As a result we find that 

 in habits, language, size, proportions, physique, and pattern of 

 coloration the two species are indistinguishable, while in color 

 alone they are different. If we take any animal (man included), 

 and endeavor to bring about a union between different species of 

 the same genus we find that in proportion as the parties to such 

 union resemble each other in habits, language, etc., as above 

 given, they will the more readily accept the situation, other con- 

 siderations being of no great importance. 



This much as regards not only the possibility but the proba- 

 bility of a voluntary union between species so circumstanced. I 

 think we must consider the interbreeding of any two species sub- 

 jected to similar conditions as not only possible but inevitable. 

 It is not in this respect that the hybrid flickers of North America 

 are unique, but in their persistent fertility and wholesale repro- 

 duction over a large area. While this instance has no parallel 

 on so grand a scale in the present history of species, so far as 

 known, it is likely that similar conditions have been and are ex- 

 erting an important influence in the evolution of life as we now 

 see it. 



If this be true, we cannot too curiously consider the relation- 

 ships of our eastern and western flickers, as time goes on, to de- 

 termine if possible the laws which govern the progress of inter- 

 breeding of species in their natural state and whether they show 

 that hybridization has any part in the evolution of new forms or 

 the extinction of the old. 



As observed, the present tendency in Colaptes resolves itself 

 into an invasion of the hardier northern race upon their counter- 

 parts of the south, with the ready absorption of the characters of 

 the former by the latter. This cross-breeding, in accord with 

 laws now recognized, should produce mixed birds superior in some 

 respects to their parents, combining the hardihood of auratus 

 with the handsomer coloration of cafer and aggressively extend- 



