time and extended observations, with the idea of learning 

 the true period of incubation, to successfully sift apart these 

 two kinds of records. 



Influence Altering the Incubation Length 



In this study it is assumed that the true length of incu- 

 bation is a blastogenic characteristic, fixed for, and as such 

 inherited by, each species ; that it is comparatively inelastic, 

 and yields exceedingly slowly to change; that with each 

 species it embraces, under optimum conditions of "tempera- 

 ture, moisture and position," a fixed minimum number of 

 days, just sufficient (and no more) to bring about the com- 

 plete development and hatching of a normal bird. A strik- 

 ing proof of its inelasticity and prepotent inheritability is 

 seen with domesticated birds, more particularly pigeons and 

 chickens ; for were this period plastic, under man's selection, 

 as are the tissues, functions and habits of these birds, one 

 would expect to find such plasticity showing itself in a 

 patent variation of the incubation period of such domesti- 

 cated birds. Man can, and has been able to cause, or flx^ 

 most extraordinary changes in his domesticated birds; with 

 pigeons, not only an increase in the number of tail feathers, 

 but even a lessening of the number of ribs (138), and with 

 chickens, not only the almost unbelievable alteration in 

 sizes from that of a bantam to a huge Cochin-China, but 

 also an increase in the number of toes (the five-toed Dork- 

 ings). Man has domesticated many other birds, and if with 

 them the period of incubation were not fixed, it seems rea- 

 sonable to believe that it should have exhibited variations 

 comparable to those variations of body, etc., mentioned 

 above, in pigeons and chickens. Yet, if I read aright, there 

 is not the slightest indication of any alteration in the incu- 

 bation period of any of man's domesticated birds; on the 

 contrary, all seem to adhere rightly to the ancestral char- 

 acteristic as shown in congeners still wild, or in wild species 

 most closely related. It has been definitely determined 

 from the experience of hundreds of poultry raisers, in 

 natural and artificial incubation, that the incubation period 

 of the domestic hen is almost exactly and almost invariably 

 twenty-one days: it matters not what breed, bantam or 

 brahma, nor however remote from, or near to, the ancestral 

 jungle fowl, the period of incubation remains the same as 

 that of the jungle fowl, viz., twenty-one days. The same 

 may be said, in effect, of the turkey, quail, pheasant, canary, 

 pigeon, duck, goose and, so far as the writer can learn, all 

 other domesticated birds. 



Furthermore, birds belonging to families having a 

 fairly similar incubation period, /. e., finches, all exhibit this 

 uniformity, even though separated by large geographic 



15 



