ON HKKEDITY. 547 



witbiu the limitatious of a common species. Siuce Charles Darwin 

 euunciated the proposition that favorable variations would tend to 

 be preserved, and unfavorable ones to be destroyed, and that the result 

 of this double action, by the accumulation of minute existiug differ- 

 ences, would be the formation of new species by a process of natural 

 selection, this subject has attained a much wider scope, lias acquired 

 increased importance, and has formed the basis of many ingenious 

 speculations and hypotheses. As variations when once they have 

 arisen, may be hereditarily transmitted, the Darwinian theory might 

 be defined as heredity modified and infiuenced by variability. 



It may be admitted that many variations which may arise in the 

 development of an individual, and which are of service to that individ- 

 ual, would tend to be preserved and i)erpetuated in its offspring by 

 hereditary transmission. But it is also without question that variations 

 which are of no service, and indeed are detrimental to the individual in 

 which they occur, are also cai)able of being hereditarily transmitted. 

 This statement is amply borne out in the study of those important 

 defects in bodily structure which pathologists group together under 

 the name of congenital malformations. The commonest form of mal- 

 formation is where an increase in the number of digits on the hands or 

 feet, or on both, occurs in certain families, numerous instances of which 

 have now been put on record. But in other lamilies there is an hered- 

 itary tendency to a diminution in the number of digits, or to a defect 

 in the development of those existing. Another noticeable deformity 

 which is known to be hereditary in some families is that of imperfect 

 development of the upper lip and roof of the mouth, technically known 

 as hare-lii) and cleft i)alate. 



These examples illustrate what may be called the coarser kinds of 

 hereditary deformity, where the redundancies or defects in parts of the 

 body are so gross as at once to attract attention. But modifications or 

 variations in structure that can be transmitted from parent to offspring- 

 are by no means limited to changes which can be detected by the naked 

 eye. They are sometimes so minute as to be determined rather by the 

 modifications which they occasion in the function of the organ than by 

 the ready recognition of structural variations. [Cases of color-blind- 

 ness, and of deaf-mutism were then referred to. | - - Dr. Horner 

 has related a most interesting family history in which color-blindness 

 was traced tiirough seven generations.* - - - Mr. David Buxton, 

 who has paid great attention to the subject of hereditary deafness,t 

 states that the probability of congenital deafness in the offspring is 

 nearly seven times greater when both parents are deaf than when only 

 one is so. In the latter case the chance of a child being born deaf is 

 less than -l per cent., in the former the chances are that 5 per cent, of 

 the children will be deaf-mutes. 



"Cited iu Die Allgimeinc, I'alholoyir, by Dr. Edwin Kleb.s, .lena, ISS7. 

 {Liverpool Medico-Cliirurg. Jottrn., July, 1857 ; January IHf/J. 



