AND OVIDUCTS IN THE AMXIOTA. 817 



Pigeons and some of the lower mammals this " milk " is squeezed 

 or injected into the young. 



The Mammalian evolution has probably gone through many 

 stages and vicissitudes, difficult to enumerate, because there were 

 many factors and not all the organs changed at the same time. 



1. We start with a hypothetical stage of Sauro-mammalia. 

 The hard-shelled egg, when laid, contains an alx-eady far advanced 

 embryo, therefore ovoviviparous. This egg was not so much 

 incubated for warmth as covered for protection by the mother. 

 The young although hatched in a reptilian condition was pro- 

 tected by the mother. 



2. Reduction of the size of the egg. Gradual preponderance 

 of extra-uterine nursing over uterine gestation, made possible 

 and introduced by the protecting insulation, in such a way that 

 an abdominal incubation area was developed, owing to the 

 reaction of the mother's abdominal surface by the sat-upon, 

 adpressed, covered egg ; this i-esulted in hypertrophic condition 

 of the cutaneous blood-vessels, hence of the glands, and correlated 

 i-eduction of hairs. Incidental change of absorption of moisture 

 through the porous shell, enhanced by loss of its calcareous 

 portion. — When this suppression had been well established, by 

 progressive inheritance, the now quite porous parchment egg had 

 the same chances of absorbing tiuid whilst still in the oviduct. 

 At the same time it stands to reason that the chances of external 

 or brood-pouch nourishing may become pi-evalent. The shorten- 

 ing of the life-period within the egg implies the birth of an 

 unripe fcetus. Foetal life must be taken as ending with the 

 bursting of the egg, no matter whether this act coincides with 

 the moment of parturition, or whether it happens some time 

 after the egg has been •' laid." In either case it coincides with 

 the cessation of any further possibility of function of yolk-sac 

 and allantois. 



We must further assume that the actual length of time re- 

 quired for the production of a young animal is the same in 

 equal-sized creatures from the beginning of segmentation until 

 it is independent. Unless we assume this, the argumentation 

 would become too complicated. 



Let us say that it took 50 days from impregnation until the 

 Sauro-mammalian youngster was ready to face the Permian 

 world. This means : 



50 days of ovoviviparous, internal uterine life (Sauro-mammal), 



40 days uterine and 10 days incubation-life within the laid egg (Hypotheria). 



30 „ „ 10 „ „ and 10 days nursing in pouch 



(Prototheria). 



20 „ „ 5 ? „ „ 25 „ „ (Monotremes). 



8 „ „ „ „ 42 „ „ (Opossum). 



The suppression of the incubation-life marks the Metatherian 

 stage just as sharply as the introduction of incubation of a 

 fcetus marked the early Mammal stage. The adaptation to the 



