494 Causes and Course of Organic Evolution 



have started a double series of mammary glands, as response 

 to the right-and-left-side coiled condition of the parent at 

 different hours. Fifteen to twenty pairs of these may have 

 formed, according to the nursing vigor of the parent, and 

 natural selection acting in connection therewith. 



Now at the present day one of the ancient marsupial groups, 

 alike as judged by distribution and structure, is that of the 

 opossums, which may have from 13 to 4 pairs of teats. The 

 teeth are numerous, usually 50 to 52. There may be no pouch, 

 or it may be imperfectly suggested by two lateral folds of 

 skin parallel to the teats, or as in the Virginian opossum it 

 may be a true pouch. The genus Myrmecohius has 4 to 2 

 pairs of teats, the teeth are 50 to 54 in number, and there is 

 no pouch though "a tract of skin shows indications of a pouch- 

 like structure" {166: 154). Both of the above belong to the 

 Polyprotodonta, or more primitive and ancient group (p. 490) 

 as we would view it. 



In SmintJiopsis, Peragale, Perameles, and Choeropus belong- 

 ing to the same group there are four to five, rarely three, 

 pairs of teats, the teeth vary from 46 to 48, and the pouch is 

 developed but opens back war dly. These however seem to 

 be derivative from forms like Phascologahy in which there 

 may be three to five pairs of teats, a like number of teeth as 

 above, but in which the only beginning of a pouch is a mere 

 skin-fold. 



In Thylacinus — a highly modified carnivorous polyproto- 

 dont — the teats are in 2 pairs, the teeth are 46, and there is a 

 backwardly opening pouch. Of other polyprotodonts Dasy- 

 uroides and one or two other genera have either no pouch or 

 it is represented by slight lateral folds. 



From evidence of fossils it can correctly be said that the 

 polyprotodonts were the world-wide and dominant marsupials 

 of late cretaceous or early eocene time, while the second group 

 of the diprotodonts were largely, perhaps wholly, australo- 

 patagonian, of later appearance, and of more specialized struc- 

 ture in nearly every important detail. Of these it may be 

 said that the teats are nearly always two pairs, in Phascolartos 

 even one pair. The teeth are on the average 38 if we take 

 account of species, but they vary from 40 through 36 to 24 in 

 Phascolomys and even 20 in the nectar- and insect-feeding genus 

 Tarsius. A complete pouch in these is nearly always formed. 



From the above and similar abundant evidence that might 

 be adduced, the writer would consider that the polyprotodonts 



