MODIFICATIONS FEOM PBESSUKE, 323 



it has been asserted that gravitation has an influence in deter- 

 mining the direction of the gi'owth of the embryo in Mammalia. 

 But it seems to have been forgotten that in most cases of 

 viviparous animals the position of the embi-yo in the uterus is 

 by no means constant, but alters in many ways during its deve- 

 lopment. If, in fact, gravitation were here of so much impor- 

 tance as has frequently been assumed, scarcely any normally 

 developed animals would be born ; for we know, from the re- 

 markably complete experiments of Marcel de Serres, that such 

 embryos as normally assume a position of equilibrium for their 

 development in the ovum — as, for instance, those of birds — are 

 invariably deformed in the most irregular manner, if they are 

 constantly moved into other positions and so the original equili- 

 brium is disturbed. In the ova of many invertebrate creatures 

 the developing embryo floats in a surrounding fluid ; so that 

 under any inversion of the whole ovary or cluster of eggs all 

 the embryos recover the same position, since the centre of 

 gravity, lying out of the centre of the body, always sinks to the 

 bottom. In such cases as these gravitation evidently acts to 

 prevent any disturbance of the equilibrium; but it is very 

 questionable whether at the same time any effect is thus pro- 

 duced on the process of development of the embryo itself within 

 the egg. That the limbs — as our arms and legs — are subject to 

 the conditions of gx'avitation allows of no doubt, and its effects 

 may very possibly tell on the same parts of the body in the de- 

 veloping embryo while stQl in the uterus. But how far this 

 influence, which, no doubt, actually exists, may contribute to 

 determine the normal formation of animals and their organs, is 

 perfectly unknown, and cannot be ascertained by any merely 

 theoretical discussion. 



Darwin adduces many instances which prove tliat even the 

 bones of the skull may be modified by gravity, or by pressure 

 in whatever manner exerted. Bums and repeated convulsions 

 in certain muscles have been known to affect the form of the 

 bones of the face ; the same effects have been produced when 

 human beings during youth have been forced to keep the head 

 constantly in a fixed position ; it is supposed that in certain 

 persons — for instance in shoemakers — who are obliged to keep 



