2/2 THE DOCTRINE OF DESCENT. 



mammals, the vascular processes and vilii of the wall 

 of the ovary become so closely connected with the foetal 

 portion of the placenta, that at birth the entire mem- 

 branous coating of the ovary is detached and thrown out 

 with it. In others, the vascular villi are not so closely 

 adherent ; they yield without important lacerations, and 

 hence no deciduous membrane (Membrana decidua) is 

 ejected. Now, as it appears to me, the specific conditions 

 of the formation of the decidua have been far too little 

 compared to justify our inferring any close affinity from 

 the mere fact that portions of the coating of the ovary 

 are lost in parturition. Much rather it must be unre- 

 servedly admitted that the formation of decidua might 

 be occasioned by subordinate circumstances of the most 

 varied kinds, and hence in orders only remotely allied, 

 or allied merely as placental mammals. We therefore 

 consider the decidua to be a subordinate systematic 

 feature where anatomical and morphological reasons 

 are opposed to it. 



We go yet further. In the modern system the form 

 of the placenta is likewise employed in the grouping of 

 organisms. If among the Deciduata, lemurs, rodents, 

 insect ivora, bats and monkeys, are classed together as 

 orders with discoidal placenta, this combination is cer- 

 tainly supported by a series of other reasons, and it is 

 quite probable that within this group of orders the form 

 of the placenta is due to homology, that is to Descent 

 But when beasts of prey, elephants and the Daman 

 (Hyrax) are further cited as orders with zonary pla- 

 centa, we find ourselves in the same position as when 

 the decidua was reckoned decisive as to the closer 

 affinity; and we are of opinion that the subordinate form 



