No. 3.] FACTORS IN THE EVOLUTION OF MAMMALIA. 389 



not inclined to accept it. Many authorities attribute just the 

 opposite influence to sexual reproduction to that which Weis- 

 mann postulates. Strasburger regards it as the means of 

 removing injurious modifications, and of keeping the species 

 constant to its type, and a very similar position is taken by 

 Hatschek and others. But in view of the many observations 

 collected by Darwin upon the subject of cross and self-fertiliza- 

 tion, and of the physiological importance of crossing to the 

 individual, both Weismann's position, on the one hand, and 

 Strasburger's, on the other, appear to be but a very partial and 

 incomplete view of the matter. At all events, we are entitled 

 to demand much fuller and more satisfactory evidence before 

 attributing to sexual reproduction such a pre-eminent place as a 

 factor in evolution. 



In the course of development the reduction and loss of 

 structures plays quite as important a part as differentiation and 

 the acquisition of new organs. No one can study any large 

 group of vertebrates without being struck by the way in which 

 the two processes accompany and balance each other. In any 

 advancing group of Organisms the course of differentiation does 

 not affect all organs alike ; some are stationary, others are pro- 

 gressing towards a higher state of efficiency, and others again 

 are retrograding and even disappearing. It is this fact which 

 makes the natural arrangement of any group of organisms such 

 a very difficult and puzzling problem. It is obvious, therefore, 

 that any system of evolutionary philosophy must include an 

 explanation of the way in which organs are reduced and sup- 

 pressed, as well as the mode in which they are acquired. The 

 current view of the matter is that the loss of organs is due 

 to atrophy from disuse, but this view is of necessity entirely 

 rejected by Weismann, who has examined the problem with 

 much care in his very interesting essay, "Ueber den Riick- 

 schritt in der Natur" (No. 59). According to him the efficient 

 cause of reduction is cessation of selection or panmixia, by which 

 useless parts, being no longer selected, tend gradually to dis- 

 appear, and a free intercrossing, as regards those particular 

 structures, is no longer prevented. This view, however, pre- 

 supposes that selection is the only power which originates and 

 maintains structures, — a thesis which is very far from being 

 proved, if it can even be regarded as probable. Indeed, the 



