REPRODUCTION AND SEX 469 



have been suggested to account for it! The best of these theories, 

 applicable to mammals only, is very interesting. Suppose a mam- 

 malian mother begins, as the result of insemination, to develop an 

 offspring, there is an ante-natal period, eleven months in a mare, 

 during which the developing embryo lives in very intimate partner- 

 ship with its mother. By means of the placenta, binding the unborn 

 offspring to its mother's womb, there is exchange of dissolved sub- 

 stances between the two. Food, oxygen, and hormones pass from 

 the mother into the embryo; nitrogenous waste-products, carbon 

 dioxide, and hormones pass from the embryo into the mother. But 

 the inheritance of the developing embryo is paternal as well as 

 maternal, and therefore the influence of some striking paternal 

 characteristics may pass back from the embryo (e.g. as hormones 

 in the blood) and influence in some specific way the constitution of 

 the pregnant mother. In a subsequent pregnancy due to another 

 father, the specifically altered constitution of the mother may 

 exert a definite influence on the new offspring; and this would be 

 telegony. This seems to be a physiological possibihty, though it is 

 more roundabout than used to be supposed. 



GRAFTS AND CHIMiERAS.— From ancient times fruit-growers 

 have practised grafting with great success. A common form of the 

 device (for it is effected in several distinct ways) is to insert a young 

 shoot of some desirable fruit-tree into the stem — often lopped — of 

 a sturdy one, taking care that the embryonic tissue (cambium) and 

 the young sap-wood of the two plants are brought firmly together 

 so that genuine physiological fusion results. The engrafted bud or 

 shoot is called the "scion", and it is made to combine with a 

 "stock". In this way a variety of fruit-tree that is marked by great 

 excellence can be quickly and surely multiplied, for the essential 

 fruiting virtues of the scion are not affected by the vigour of the 

 sturdy stock, though its vegetative growth may be checked. But 

 even this checking is an advantage when the object in view is to get 

 much fruit. Thus we see fine orchards with the high-class engrafted 

 young trees bearing very abundant and conveniently reachable 

 fruit on the shoulders of the inconspicuous old-fashioned stocks. 

 Scion and stock must be near relatives; thus the peach may be 

 grafted on a plum stock, the apricot on an almond, the apple on 

 a pear, the pear on a quince, the medlar on a hawthorn, and so on. 

 In modern times grafting of succulent plants has been successfully 

 effected, but most grafts are between trees. It is possible that the 

 idea of grafting was suggested to the early fruit-growers by noticing 

 that two branches of the same tree, or even of adjacent trees, some- 

 times fuse when they are so much rubbed together by the wind 

 that two abraded surfaces are formed, and a mending of the mutual 

 wounds results in a thoroughgoing fusion. 



