254 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, 1890. 



species is a mistake has been thoroughly established by Darwin, who 

 has shown clearly what previously had been known, but not scienti- 

 fically expressed, that the animal organism is " plastic in the hand 

 of man." Nature seldom resorts to abrupt changes in development; 

 usually progress is gradual in the extreme, and the results the more 

 sure. One of the essentials to organic development is a limitation 

 of the power of inter- crossing ; and the germ of the physiological, 

 anatomical, and other characters of a species opposes the production 

 of distant hybrids. However, no two individuals are alike ; variety 

 does not appreciably impede crossing ; and the allied species of a 

 group even are capable of producing hybrids. There is no abrupt 

 line to be drawn by science in this matter of crossing, no hard and 

 fast boundary, but there is a tide mark of a certain range, and 

 within the limits of that range the result of crossing is doubtful ; 

 inside the range crossing is certain ; outside it is impossible, although 

 certain most extraordinary "yarns" of crossing most diverse species 

 have from time to time gained acceptance from the public, and 

 occasionally from scientific workers. Crosses actually occur in 

 nature. For example, Prejevalsky in Thibet saw among other wild 

 equines, mules, and his observations agree with those of previous 

 travellers, and should permanently dispel the view that mule-breeding 

 is unnatural and contrary to divine laws, which is occasionally an 

 impediment to its expansion in this country and elsewhere. 



The infertility of hybrids is well exemplified in the case of mules, 

 for it is almost invariably a fact with them. Although a few cases are 

 on record in which the he-mule has begotten progeny and the 

 female has come in foal, I know of no authenticated instance of 

 a foal being born with a mule for its sire and another mule for its 

 dam. " When mules breed," was the favourite expression among the 

 Roman soothsayers to indicate hopelessness as to the issue of occur- 

 rence of events, and yet in history there have been occasions when 

 mules did breed. These are for the most part pure fables, but 

 they indicate a general impression that such an extraordinary event 

 does occasionally come off, now and again in a long lapse of ages. 



One of the most energetically discussed problems of the present 

 day is the question of degrees of hereditary transmission from the 

 parents respectively and from other ancestors. It is a matter of 



