^'°1908^^] ^^'^^^^ Literature. 331 



' up in turn "the homesick theorj''," "the desire to disperse theory," "the 

 nestHng food theory," " the safe nesting theory," all of which are given short 

 shrift,- mostly with reason. Some half-dozen "ancestral-habit" theories 

 are also cited and summarized. Many of the postulates attributed to the 

 authors mentioned did not, however, originate with them nor at the dates 

 implied, but were of much earlier origin and in a measure common property 

 long before the implied dates. The principal factor put forward by Marek 

 in 1906, that of the influence of barometric pressure — birds migrating 

 from areas of high barometric pressure to areas of low barometric pressure 

 — was stated in substance, and nearly in the same terms, by Cooke a dozen 

 years before; and so with the main points of other recent theories here sum- 

 marized, some of which were brought out by American writers twenty years 

 before the implied date of origin here given. This is not said in disparage- 

 ment of the later authors cited by Mr. Walter. For example, Marek's 

 explanations of how and why birds migrate is based on his own independ- 

 ent and extended original investigations of the movements of birds in 

 Europe, and is none the less interesting and valuable because it is in the 

 main confirmatory of earlier investigations and conclusions made else- 

 where, and for many years more or less generally accepted by those who 

 are best acquainted with the real facts of migration. Thus, Walter says: 

 " From his [Marek's] point of view there is no necessity for referring the 

 habit of migration to hypothetical ancestral behavior, nor for endowing 

 birds with such himian attributes as love of home or the memory of pre- 

 vious successes. The streaming northward of birds in the spring and their 

 return southward in the fall are both primarily dependent upon the same 

 observable external factors as those which cause the flow of the air in the 

 form of prevailing -ninds, northward in the spring and southward in the 

 faU." 



While no facts in relation to the habits and beha^•ior of animals are in 

 the main better established than the above, Mr. Walter is able to see only 

 "an immense halo of mystery around bird migration." This is perhaps 

 due to his having overlooked a principle of prime importance, or to which 

 at least there is no allusion in his very interesting smtmaary of the sub- 

 ject. This is the intimate interrelation of the impulse of migration and 

 the fimction of reproduction. As we stated the case some fifteen years 

 ago: "If we consider that migration consists really of two movements — 

 that is from the breeding station to the winter quarters and then back 

 again — and that the one movement is the necessary complement of the 

 other, it is hardly necessary to seek for a separate cause for the two move- 

 ments; the t\AO together constitute migration in a complete sense, which, 

 as already explained, is an inherited habit, — an inherent, irresistible im- 

 pulse, closely blended -nith the function of reproduction. The promptings 

 which lead to the migratory movement, respectively in fall and spring, 

 have unquestionably a different origin; the autmnnal movement being 

 doubtless [at least primarily] prompted by a reduction of temperatui'e and 



