820 DR. HANS GADOW ON OVARIES 



To judge from analogy the new organ should continue straight 

 on, leading in this case through a diff'use and cotyledonary to a 

 solid, and from a nDn-deciduous to a deciduous placenta, ter- 

 minating in the birth of the young in an utterly helpless con- 

 dition. As a rule the non-deciduous and diffuse placenta is found 

 in those mammals which at birth require least maternal care. 

 The latter implies greater mental capacity. But we also know 

 from analogy that a recent invention — although it may be 

 capable of much perfection — is not always kept up if the old 

 string to the bow is still capable of further adaptation. Where 

 both alternatives are good, workable, and ca.pable of being 

 improved, it may take a long time to settle which is after all the 

 better of the two. 



"What ha,s induced some Metatheria to neglect the further 

 development of their placenta and to intensify the marsupial 

 alternative ? The morphological and physiological momentum 

 was sxirely on the side of improving placentation. There 

 must have been some external, environmental influence, and 

 it was Dollo's brilliant suggestion that arboreal life was the 

 underlying cause. His further explanation is less acceptable, 

 that the climbing habits have caused premature births, Avhich 

 have become habitual, and that the hapless foetus had therefore 

 to be nursed and located in a marsupium, the bursa, or rather 

 multiple bursae, being no longer sufficient. 



I think the new conditions to be faced by intense arboreal 

 life have acted somewhat differently, especially if, as we assume 

 now, all the Metatheria were already possessed of a marsupium*. 



I venture to suggest that those Metatheria which were driven 

 to arboreal life, had to solve the question what to do with 

 their young. They had to carry them, and being already pos- 

 sessed of the pouch, this was not only retained but intensified 

 by prolonged use. That the facilities offered by such a pouch 

 are great, is shown by the still existing numerous and much 

 diversified Marsupials, and by the fact that some at least, e. g. 

 Opossum, have reduced, the uterine gestation almost to the 

 conceivably lowest limit. Whilst these arborealised Metatheria 

 could not afford to leave their young behind, it was different 

 with those other Metatheria which duiing the Cretaceous epoch 

 somewhere "in the larger and more effective workshops of the 

 North" (to apply here one- of Darwin's happiest expressions) went 



* Klaatsch's view of the correlation between Bursas and Marsupium had to be 

 considerably modified bj' Semon's discovery that the Monotreme does not lie in a 

 bursa but in an already typical, though transient, marsupium. Moreover, there are 

 unmistakable vestiges of marsupial muscles, other than compressores mamma", 

 in various Placentals. Lastly, each teat with its areola does represent a bursa 

 (Klaatsch), but if each bursa had been a bro6d-pouch instead of merely the result of 

 the attachment of young, the possession of a series of bursas from near the vulva 

 up to the armpit would imply an impossible condition. In fact, bursse or nipples 

 can be formed independent of a marsupium and in places whereto no such organ 

 can have extended. 



