EVOLUTION 



1075 



development in insects like blow-flies, whose eggs are laid in flesh, 

 is adaptive to the transient nature of that habitat ; though it is not 

 inconsistent with this interpretation to recognise the stimulating 

 quality of the medium towards rapid growth. Or, again, the lengthen- 

 ing out of the ante-natal period — ending in viviparous birth — is an 

 adaptation against the chances of death among delicate offspring. 

 Even such subtle characters as the length of life, the age at which 

 reproduction occurs, the localisation of the regenerative capacity, 

 may be interpreted as adaptive, as we have sought to show else- 



Saw of Sawfish (Pristis), an exaggerated prolongation of the anterior end of 

 the cartilaginous skull, probably of most use as a weapon. From a 

 specimen. 



where. As to adaptations before birth, they are most appropriately 

 referred to the previous groups — either structural or functional. 

 Thus the diffusion of hormones through the mammalian placenta, 

 from mother to offspring and perhaps vice versa, is an ante-natal 

 functional adaptation. Similarly, the sheath of cartilaginous tissue 

 that covers the saw of a sawfish embryo before birth, and pre- 

 vents injury to the wall of the oviduct is a very useful ante-natal 

 adaptation of structure. In a female sawfish (Pristis cuspidatits) , 

 15 J feet long, Southwell found twenty- three embryos in the oviducts. 

 Each of these had about nine inches of body-length and other five 



