AND OVIDUCTS liST THE AMNIOTA. 821 



ill for that further mental development which is intimately 

 correlated with the origin of the corpus callosum. Competition is 

 most effective if backed by brain-power. Where these dawning 

 Eutheria spread the other Metatheria had to go to the wall, or 

 were forced to get out of reach — into the trees. Those which, 

 already arborealised, found their way into countries like Australia, 

 where the Eutheria could not follow in time, found there 

 nothing to prevent them from becoming again terrestrial, as 

 Dollo has shown so conclusively. The chance of reviving the 

 degenerated placenta was, however, gone, and the Marsupials are 

 now, in spite of the retention of various low characters, the most 

 specialised subclass of Mammals. 



The Edentates present a striking analogous case : being with- 

 out exception arboreal or fossorial, they also seem to have got 

 out of the way of mentally higher, dangerous Eutheria. They 

 have survived, whilst those that went in for heavy bulk and 

 armour were not a great success. At least the American Eden- 

 tates seem to have come into contact with the dangerous 

 Eutheria long after they themselves had become typical Eutheria, 

 so that the former possession of a marsupium could avail them 

 nothing. 



_A few words about the Eutheria. They could afford to part 

 with theii^ young for a time and to suckle them intermittently, 

 an advantage which needs no further comment than that it 

 would be impossible without higher mental faculties. 



Great adaptive changes have been wrought ; every chance has 

 been tried, now here, then there. Nearly all the Non-deciduata 

 are at birth precocious, able to look after themselves, no matter 

 whether the placenta is diffuse, cotyledonary or zonary even, 

 namely the aquatic Cetacea and Sirenia, and the Artio- and 

 Perissodactyles, the exception being the Lemurs. The Deciduata 

 are at birth helpless : Kodents, Insectivores, Carnivores, Chiro- 

 ptera, Tarsius and Simile, except the Elephants. 



The egg- laying Monotremes, the implacental Marsupials, and 

 the Mammals with the most advanced placenta, are born in a 

 condition as helpless as the highest nidicolous birds, whilst the 

 avowedly low non-deciduous and diffuse placenta of the Mare i 

 sufficient to produce a young which is as precocious and in- 

 dependent as a lizard. 



Can these facts be brought into line ? What is here primitive 

 and what is a roundabout return to apparent but not really 

 ancestral conditions ? 



Prog, Zool. See— 1912, No. LV. 55 



