34 THE EVOLUTION OF SEX. 



§ 3. The theory that there are two kinds of ova, respectively 

 destined to develop into males or females, is more than a 

 mere begging of the question. The constitution of the ovum 

 is undoubtedly a fact of primal importance, but we must also 

 recognise that what is virtually decided at this early stage may 

 be counteracted by later influences of an opposite character. 

 The hypothesis of two kinds of o\'a was advanced, for example, 

 by B. S. Schultze, but as the grounds for his views are not 

 admitted as correct, only its existence need be noticed till more 

 observations are forthcoming. 



§ 4. Numerous authors have attached great importance to 

 the process of fertihsation as a determinant of the sex. 



One of the most crude positoins has been that of Canestrini, who 

 ascribed the determination of sex to the number of sperms entering the 

 ovum : — The more sperms, the greater the tendency to male offspring. It 

 has, however, been shown by Fol, Pfliiger, Hertwig, and others, that 

 "polyspermy," or the entrance of more than one sperm, is extremely rare, 

 is in fact generally impossible, and when it does in rare conditions occur, 

 indicates a pathological condition of the egg-cell, and tends to produce 

 abnormalities. Pfliiger diluted the seminal fluid of male frogs, and 

 found that no change resulted in the normal numerical proportion of 

 the sexes. The case of drones, furthermore, where male are known to 

 arise from unfertilised ova, is a familiar example, exactly counter to 

 Canestrini's proposition, which may in fact be dismissed as wholly 

 untenable. 



§ 5. Time of Fe7-tilisatio7i. — ^Yith greater weight various authorities 

 have insisted upon the time of fertilisation. Thus, according to Thury 

 (1863), followed by Diising (1S83), an ovum fertilised soon after liberation 

 tends to produce a female, while an older ovum %\-ill rather develop into a 

 male. As a practical breeder Thury claimed to determine the sex of cattle 

 upon this principle ; Cornaz and Knight have both practically confirmed 

 this ; while Girou has pointed out, that female flowers fertilised as soon as 

 they were able to receive pollen tended to produce female offspring. 

 Hertwig has also shown that, the internal phenomena of fertilisation vary 

 somewhat with the age of the ovum at the time. Hensen is inclined to 

 accept the general accuracy of Thury's conclusion, but extends it to the 

 male element as well. "A very favourable condition in both ovum and 

 sperm will probably lead to the formation of a female." " According to 

 its condition, a sperm may either insufficiently corroborate the favourable 

 state of the ovum, or constitutionally strengthen an ovum less satisfactorily 

 conditioned." 



§ 6. Age of Parents. — Hofacker (1823) and Sadler (1830) independently 

 published a body of statistics, each including about 2,000 births, in favour 

 of the generalisation that when the male parent is the older the offspring 

 are preponderatingly male ; while if the parents be of the same age, or 

 a fortio7-i if the male parent be the younger, female offspring appear in 

 increasing majority. This conclusion, generally known as Hofacker's and 

 Sadler's law, has received both confirmation and perplexing contradiction. 

 It has been confirmed by Gohlert, Boulenger, Legoyt, and others, and by 



