68 SECTIONAL ADDEESSES. 



animal's gradual development from the embryo to the adult, and there- 

 fore prove nothing. Even now there are those who maintain that the 

 continuity of the germ-plasm is inconsistent with any true recapitula- 

 tion. Let us try to see what this means. Take any evolutionary series, 

 and consider the germ-plasm at any early stage in it. The germ, it is 

 claimed, contains the factors which produce the adult characters of that 

 stage. Now proceed to the next stage of evolution. The germ has 

 either altered or it has not. If it has not altered, the new adult 

 characters are due to something outside the genn, to factors which may 

 be in the environment but are not in the germ. In this case the animal 

 must be driven by the inherited factors to reproduce the ancestral form ; 

 the modifications due to other factors will come in on the top of this, 

 and if they come in gradually and in the later stages of growth, then 

 there will be recapitulation. There does not seem to be any difficulty 

 here. You may deny the term ' character ' to these modifications, and 

 you may say that they are not really inherited, that they will disappear 

 entirely if the environment reverts to its original condition. Such lan- 

 guage, however, does not alter the fact, and when we pass to subsequent 

 stages of evolution and find the process repeated, and the recapitulation 

 becoming longer, then you will be hard put to it to imagine that the 

 new environment produces first the effects of the old and then its own 

 particular effect. 



Even if we do suppose that the successive changes in, say, an 

 ammonite as it passes from youth to age are adaptations to successive 

 environments, this must mean that there is a recapitulation of environ- 

 ment. It is an explanation of structural recapitulation, but the fact 

 remains. There is no difficulty in supposing an individual to pass 

 through the same succession of environments as were encountered in 

 tiie past history of its race. Every common frog is an instance. The 

 phenomenon is of the same nature as the devious route followed in their 

 migrations by certain birds, a route only to be explained as the repetition 

 of past history. There are, however, many cases, especially among 

 sedentary organisms, which cannot readily be explained in this way. 



Le*: us then examine the other alternative and suppose that every 

 evolutionary change is due to a change in the germ — how produced we 

 need not now inquire. Then, presumably, it is claimed that at each 

 stage of evolution the animal will grow from the egg to the adult along 

 a direct path. For present purposes we ignore purely larval modifications, 

 and admit that the claim appears reasonable. The trouble is that it 

 does not harmonise with facts. The progress from youth to age is not 

 always a simple advance. The creature seems to go out of its way to 

 drag in a growth-stage that is out of the straight road, and can be ex- 

 plained only by the fact that it is inherited from an ancestor. Thus, 

 large ammonites of the Xipheroceras planicosta group, beginning 

 smooth, pass through a ribbed stage, which may be omitted, through 

 unituberculate and bituberculate stages, back to ribbed and smooth 

 again. The anal plate of tlie lan^al Antedon, which ends its course 

 and finally disappears above the limits of the cup, begins life in that 

 lower position which the similar plate occupied in most of the older 

 crinoids. 



