194 PROCEEDINGS OF THE ACADEMY OP 



carried. There is, I believe, a perfect parallelism in the physiological point 

 involved between mammals and birds ; but it does not reach to the grade just 

 supposed. In Marsupials, the condition of the young at birth is associated 

 with a radical modification of the generative apparatus in both sexes. The 

 sexual organs alone would enable us to predicate, regarding the condition of 

 the foetus at term, a departure from the ordinary standard obtaining in higher 

 orders, even were we unable to say in what the difference would consist. It 

 is not so with birds ; if there be any structural modifications correspondent to 

 altricial and praecocial nature, they remain to be discovered. From nothing 

 now known could either be predicated, in any given instance ; our inferences 

 would be purely upon consideration of known allied forms. 



Marsupials are so far " synthetic " that they comprehend the semblances of 

 other mammalian orders. There are carnivorous, herbivorous, quadrumanous, 

 etc., marsupials. The number of forms that a given group may represent or 

 repeat under modifications peculiar to itself, is a well-known test of the char- 

 acter of such assemblage, and in a measure of its grade in a taxonomy, only 

 inferior to that afforded by the modifications themselves. Marsupials and 

 Monotremes conform in both ways to the requirements of a primary division 

 of mammals. But among birds, altricial and prsecocial orders must be so 

 drawn that, while they are mutually representative to a degree, as wholes, they 

 do not include, each in itself, modified representations of each other. Thus, 

 e. ff., in Bonaparte's parallel of altricial Gavix and prfficocial Anseres, among 

 swimmers, it is impossible to find families representing each other to anything 

 like the degree, or in anything like the number of cases, that may be shown 

 when Natalores are divided upon another basis, and the families differently 

 worked out. 



If helplessness at birth, compared with precocity, means, among birds, 

 " high " as opposed to " low " in the scale, then either the reverse is the case 

 with mammals, or else we must compare altricial Insessores with marsupials, 

 and prsecocial Natatores with the higher orders : a dilemma, either horn of 

 which is sufficiently difficult. 



I believe that the strongest analogy, if not actual homology and exact 

 parallelism, between the altricial and praecocial series of birds, and certain 

 conditions of mammals, really does exist. My meaning will be evident, if I 

 simply refer to the difference between a kitten or puppy at birth, and a newly- 

 born calf or colt. If all mammals are "altrices" by their very nature, as ex- 

 pressed in their name, there is a distinction with a difference in the two modes 

 of exhibiting this nature. The young of one series of mammals are born weak 

 and helpless, unable to stand, often blind and naked ; the nipple must be 

 brought to these. In the other series, the young walk or swim at birth, are 

 clothed like the parents, and can see ; these betake themselves to the nipple. 

 Here is the mammalian representation of the two avian series. This is so 

 evident and so certain that it must have been already recognized, though it is 

 original so far as the writer is concerned.* As among birds, moreover, this 

 distinction marks, iu a general way, large groups ; even orders might be based 

 upon it. But it would not suffice for a primary dichotomous division of mam- 

 malia, even of placental mammals ; nor, should occasion arise, as when its 

 requirements did not accord with structural modifications, would it take pre- 

 cedence over the latter, to the dismemberment or inversion of natural groups. 



In opposing undue estimate of the value of this physiological character, I 

 am far from underrating, still less ignoring, its import. As collateral testimony 

 in the formation of orders and location of families, it has much weight; and 



*I find exact expre.ssion of the case: — " The differences in degree of individual develop- 

 ment attained at l)irtli parallel, in Mammals, those in Birds expressed by the terms altrices 

 Hnd prucoce.s. The hoofed quadruped enters the world with the use of all its .senses; in 

 a few hour.s can follow the dam, and keep pace with her if she sees cause for flight: the 

 feline is born Mind and helpless; some days elapse ere the commissure of the eyes is un- 

 sealed."— Owen, on membrana impillaris, Comp. Anat. and Phys. Vert, iii, p. 754, 1808. 



[Dec. 



