126 OUTLINES OF EVOLUTIONARY BIOLOGY 



How this intimate correlation between gonads and secondary 

 sexual characters is brought about is still uncertain, but there is 

 strong reason to suppose that it is due to the secretion by the 

 gonad of some specific substance (hormone), perhaps of the 

 nature of a ferment, which circulates throughout the body 

 chiefly no doubt in the blood and controls the development 

 of the characters in question. That internal secretions may act 

 in this way upon organs remote from their own place of origin is 

 well known and is strikingly exemplified in the case of the 

 corpora lutea of the mammalian ovary. These structures appear 

 on the surface of the ovary in the places whence ova have been 

 discharged, and are apparently of a glandular nature. As the 

 fertilized ova pass down the oviduct they begin to develop, and 

 on reaching the uterus fix themselves to the wall of the latter, in 

 which they become imbedded. The spot where fixation takes place 

 is far distant from the ovary, but it has been demonstrated that 

 if the corpora lutea on the surface of the latter be destroyed the 

 embryos will not become fixed at all. There is a close correlation, 

 then, between the presence of the corpus luteum and the fixation of 

 the embryo, and this is explained by supposing that the corpus 

 luteum secretes some substance which, circulating in the blood, 

 reaches the uterus and stimulates its epithelial lining to respond 

 to contact with the embryo. Moreover, the reaction, whatever its 

 cause, appears to be mutual, for if the discharged ovum does not 

 get fertilized the corpus luteum does not attain its full develop- 

 ment and soon disappears. 



We have now very briefly traced the evolution of sexual 

 characters from their starting point in the male and female 

 gametes of the Protista to their culmination in the higher plants 

 and animals. We have seen how sexual differentiation, which 

 primarily concerns the gametes themselves, is gradually extended 

 to the colonies or to the multicellular individuals from which the 

 gametes arise, or even to a preceding, originally non-sexual 

 generation. The various structural modifications thus brought 

 about are all directed towards one end, the conjugation of the 

 gametes. The mutual attraction which undoubtedly exists 

 between the gametes themselves is not sufficient, at any rate in 

 the case of the more highly developed and complex organisms, 

 where the distances between their places of origin are relatively 

 very great, to secure their union. 



In the flowering plants their own efforts are supplemented by 



