1 899] MEHNERT'S PRINCIPLES OF DE VEL OP ME NT 3 S 7 



canal in the higher vertebrates, and the late occurrence of cartilag- 

 inous connections between pieces of skeleton which arise isolated from 

 one another, e.g. the connection which Wiedersheim demonstrated 

 between the head of the femur and the pelvis in amphibians, are other 

 cases in point. 



After discussing the above instances in detail the author gives a 

 tabular summary, the general result of which is, that phylogenetically 

 progressive organs exhibit a general acceleration in their rate of 

 development, while retrogressing organs tend to be more and more 

 belated. A structure whose racial importance is waning cannot be 

 simply suppressed ; a structure whose racial importance is increasing 

 cannot be simply exaggerated ; there is no question of per se lopping 

 off or tacking on ; the embryological facts point to " an alteration 

 of the entire course of development from the first appearance of the 

 primordium (rudiment, Anlagc) to the cessation of the process." This 

 conception may perhaps be harmonised with Weismann's theory of a 

 germinal struggle between the waxing and the waning determinants 

 (or let us say structural units), — an unseen struggle within the 

 germ-cell whereby an intra -selection abets the ordinary process of 

 natural selection. The determinants which are in force in the germ 

 which secure success in nutrition and otherwise, will have a free 

 course and an accelerated development. Having illustrated his 

 central thesis, — that the rapidity of the growth of an organ is 

 proportional to its degree of development, Mehnert proceeds to further 

 analysis. 



He first defines abbreviation as a premature cessation of develop- 

 ment, illustrated for instance by numerous skeletal parts which do not 

 complete their natural histological differentiation, — a cartilaginous 

 metastyloid in a man fifty years of age, a cartilaginous epipubis in an 

 ostrich twenty-five years old, or the more or less persistently carti- 

 laginous sternum of certain cetaceans. From this " abbreviation " he 

 distinguishes " retardation " or the slowing of the developmental process, 

 as in the case of a wisdom tooth. Abbreviation is the insertion of a 

 full stop at an early stage ; retardation is slow growth long drawn out. 

 But the two modes of retrogression may be combined. 



Similarly, in regard to progressive structures, the author defines 

 acceleration or the quickening of developmental processes, which he 

 illustrates chiefly by comparing the different rates of growth of various 

 skeletal parts during foetal life ; and prolongation or the lengthening 

 of progressive development, which he illustrates by the earlier and 

 earlier appearance of certain primordia (e.g. of metacarpals and meta- 

 tarsals), and by the extension of the growing period without loss in rate. 

 But, again, the two modes of progression may act in concert. 



In the turtle the long bones, such as humerus, femur, radius, and 

 ulna, arise from primordia which differ but slightly from the average 

 size of a carpal or phalanx rudiment. But although they start very 



