124 RETROGRESSION 



former. Still better examples are seen in the disappearance in 

 later life of structures which are useful to the developing individual, 

 but which have no place in the scheme of adaptations of which the 

 adult is compacted. It is because of " selection which affects not 

 increase of an organ but decrease of it," that the human being 

 loses the placenta, the frog the special structures of the tadpole, 

 and many insects their larval organs. To sum up, cessation of 

 selection causes real retrogression, a real loss to the race (i.e. to 

 the germ-plasm) of ancestral characters ; reversed selection, on the 

 other hand, though it sometimes collaborates with cessation of 

 selection to produce real loss, causes, as a rule, like ordinary selec- 

 tion, not loss but gain to the germ-plasm, to which it adds a 

 hereditary tendency. It does not abbreviate ; on the contrary it 

 lengthens the life-history. Speaking generally, it causes the loss in 

 the later stages of development of characters which have developed 

 earlier. That is its special, its more common function. 



205. In the present work we have accepted Weismann's theory 

 of the continuity of the germ-plasm. That theory fits the facts so 

 well that it has very few opponents at the present day. It 

 supposes that the germ-plasm is not formed afresh in every germ- 

 cell, but, by means of cell-divisions, is handed on by germ-cells 

 to descendant germ-cells. The process is a continuation of that 

 which occurred in the unicellular ancestors. The germ-plasm varies 

 and, since it is alive, its chemical constituents (carbon, oxygen, 

 nitrogen, etc.) change in a constant stream. Nevertheless from age 

 to age it is continuous in the same sense as the human body is 

 continuous from day to day. 1 In addition we have supposed, 



1 As far as I am able to judge, this, in essence, is the form in which Weismann 

 wishes us to accept his theory. He writes in his most recent work, " The 

 expression (ancestral plasm), however, has been very frequently misunderstood, 

 as if it were intended to mean that the ids retained unchanged for all time the 

 characters of their respective ancestors ; and I have even been credited with 

 supposing that our own ids still consist of the determinant-complexes of our 

 fish-like or amoeba-like ancestors. But in reality no id exactly or completely 

 corresponds to the type, that is to the whole being of any one of the ancestors 

 in whose germ-plasm it was formerly contained, for each of the ancestors had 

 many ids in his germ-plasm, and his entire constitution was not determined 

 by any one of them alone, but by the co-operation of them all. . . . Thus, accord- 

 ing to our view, the germ-plasm consists of ids, each of which contains all the 

 determinants of the whole ontogeny, but usually in individually different 

 quality " (The Evolution Theory, Eng. Trans., vol. ii. p. 38). Weismann's con- 

 ception of the germ-plasm, then, appears to be of a substance containing discrete 

 particles which he terms ids, any one of which is capable of directing development. 

 The ids differ from one another, but only in so far as is implied by the variations 

 of individuals. As noted by him, the term ' ancestral ' has resulted in con- 

 fusion ; it is inaccurate and misleading. TheTconception of the germ-plasm 



