39 o /. ARTHUR THOMSON [may 



his learning and experience to good account when he argues that in 

 each species there is an individuality- — eine Specificitat — in develop- 

 ment. And since there is variability each development is literally a 

 " Kainogenesis." This is one of the strongest sections in the book. 

 Mehnert goes on to point out that an acquired character such as the 

 strength of an athlete's biceps has to be sustained by a continual repeti- 

 tion of the original stimuli which produced it, whereas congenital 

 characters are at least represented, though not fully developed, even 

 when they do not receive their appropriate functional stimulus. Per- 

 sistence and length of life depend primarily on the constitution, there 

 is only indirect gain in the acquired. It may save the life of the 

 individual in struggle ; it may, according to the modern doctrine of 

 indirect selection or " organic selection," save the race until a con- 

 stitutional variation in the same direction is evolved. But to this 

 consideration Mehnert adds his particular theory of cumulative energy. 

 The life of a species runs on a principle of compound interest ; there 

 is a cumulative augmentation of the mysterious specific developmental 

 energy. 



The author makes a brave attempt to keep faith with the recapitu- 

 lation doctrine, and with himself. He admits the individuality of 

 development ; he admits that the order of development in ontogeny is 

 often the reverse of the order of evolution in phylogeny ; he admits 

 that we have to look for a recapitulation of stages in organogenesis 

 rather than of stages of entire organisms, as Dr. Beard has also main- 

 tained ; but he clings fondly to the old conclusion, provided it be cor- 

 rected by the notion of a variable rate of growth. There is a sugges- 

 tion here of the old shoe cobbled so often that there is nothing of the 

 original left. 



In the section on the degeneration or involution of organs, Mehnert 

 points out that this is, so to speak, normal. The organism is an inte- 

 grate of parts which have different rates of development. It is like 

 the march of a great procession ; some members are continually falling 

 behind, others are always shooting on ahead. Involution and evolu- 

 tion go hand in hand. The ovum develops, the polar bodies degenerate ; 

 the blastoderm develops, the merocytes degenerate ; the head grows 

 and the tail dwindles; the higher nerve-centres of the Mammalian 

 brain increase and the occipital region diminishes ; the metanephros 

 comes and the pronephros goes ; the backbone strengthens and the 

 notochord disappears ; and so on throughout, a continual necrobiosis. 



Mehnert's essay might be in part described as a recognition of the 

 time-element in development. The material of evolution is slow to 

 change in kind — it is the germ-plasm — but its rate, and all matter 

 has its rate, is more readily altered. We may not transmute the 

 metals, but we can turn gas into fluid or solid at will, i.e. we can 

 change the rate of molecular movement. 



Thus our author ends fitly by expounding the effect of altering the 



