100 



afforded the passage from the Metatherian to the Eutherian stand, and 

 it led to the loss of the marsupium as a structure no longer required. 



This type of gestation of two units is still in existence in many 

 of the lower Eutheria, as mouse, rabbit, etc., and probably in mole and 

 hedgehog, A gestation of two units, although it gave an advance on 

 the condition at birth characteristic of forms, whose gestations were 

 made up of one unit only, did not yield the advantages to be gained 

 by a still further prolongation of the gestation to include three or 

 more critical units. These advantages were seized upon in course of 

 time by various mammals, and utilised to varying extents, their 

 gestations gradually coming to include a greater number of critical 

 units. 



But, wherever further prolongation was acquired, it was always 

 still effected under the rules originally laid down, as to a correspondence 

 between the length of gestation and a certain number of critical and 

 ovulation-units. 



By a curious coincidence, for it is nothing more, in the table of 

 gestation-lengths, critical units, etc., given up in the complete work, 

 forms with from one to eight critical units in the gestation are in- 

 cluded. An examination of the list will make it clear, that, as a rule, 

 forms with a greater number of such units in the gestation are born 

 in a condition more advanced than those with a less. Mere length 

 of gestation does not effect this, for a mouse attains in 20 days a 

 degree of development as advanced as a rabbit in 30, or as a dog in 

 62 : or, again, an opossum reaches in 8 days the stage of development 

 arrived at by a rabbit in 15 days, a cavy in 22, a sheep in 30, a 

 horse in 42, or a human embryo in 46 days. 



The reason of a greater degree of development at birth with an 

 increased number of critical units lies deep, and is to be sought in 

 the circumstance, that forms with gestation-periods of several critical 

 units must in their ancestral history have passed through conditions, 

 in which the number was smaller. They must have started with a 

 gestation of one critical unit , and have gradually increased this 

 number. 



While many of the lower Eutheria still maintain very conservative 

 traditions, among the higher ones in certain orders, thus in Ungulata 

 and in man, a rather different state of affairs obtains, probably as a 

 secondary condition. Even in the lower forms, such as rabbit and 

 cavy, it is rendered likely, from a comparison with the short critical unit 

 of the opossum or Hypsiprymnus, that the critical unit, and with 

 it the ovulation-unit, has been doubled or trebled. 



