September 17, 1920] 



SCIENCE 



263 



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 harmonize 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 explained only 

 by the fact that it is inherited from an an- 

 cestor. Thus, large ammonites of the Xiphe- 

 roceras planicosta group, beginning smooth, 

 pass through a ribbed stage, which may be 

 omitted, through unituberculate and bituber- 

 culate stages, back to ribbed and smooth again. 

 The anal plate of the larval 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. 



Here, then, is a difficulty. It can be over- 

 come in two ways. A view held by many is 

 that there are two kinds of characters: first, 

 those that arise from changes in the germ, 

 and appear as sudden or discontinuous varia- 

 tions; second, those that are due to external 

 (i. e., non-germinal) factors. It seems a corol- 

 lary of this view that the external characters 

 should so affect the germ-plasm as ultimately 

 to produce in it the appropriate factors. This 

 is inheritance of acquired characters. The 

 other way out of the difficulty is to suppose 

 that all characters other than fluctuations or 

 temporary modifications are germinal; that 

 changes are due solely to changes in the con- 

 stitution of the germ; and that, although a 

 new character may not manifest itself till the 

 creature has reached old age, nevertheless it 

 was inherent in the germ and latent through 

 the earlier growth-stages. This second hy- 

 pothesis involves two further difficulties. It is 

 not easy to formulate a mechanism by which 

 a change in the constitution of the germ shall 

 produce a character .of which no trace can be 

 detected until old age sets in; such a char- 

 acter, for instance, as the tuberculation of the 

 last-formed portion of an ammonite shell. 

 Again, it is generally maintained that char- 

 acters due to this change of germinal factors, 

 however minute they may be, make a sudden 



appearance. They are said to be discontinu- 

 ous. They act as integral units. !N'ow the 

 characters we are trying to explain seem to us 

 paleontologists to appear very gradually, both 

 in the individual and in the race. Their be- 

 ginnings are small, scarcely perceptible; they 

 increase gradually in size or strength; and 

 gradually they appear at earlier and earlier 

 stages in the life-cycle. It appears least diffi- 

 cult to suppose tha't characters of this kind are 

 not initiated in the germ, and that they, if no 

 others, may be subject to recapitulation. It 

 may not yet be possible to visualize the whole 

 process by which such characters are gradu- 

 ally established, or to refer the phenomena of 

 recapitulation back to more fundamental prin- 

 ciples. But the phenomena are there, and if 

 any hypothesis is opposed to them so much the 

 worse for the hypothesis. However they be ex- 

 plained, the instances of recapitulation afford 

 convincing proof of descent, and so of genetic 

 evolution. 



THE " LINE UPON LINE " METHOD OF PALEON- 

 TOLOGY 



You will have observed that the precise 

 methods of the modern paleontologist, on which 

 this proof is based, are very different from the 

 slap-dash conclusions of forty years ago. The 

 discovery of Archmopteryx, for instance, was 

 thought to prove the evolution of birds from 

 reptiles. E'o doubt it rendered that conclusion 

 extremely probable, especially if the major 

 premiss — ^that evolution was the method of 

 nature — were assumed. But the fact of evolu- 

 tion is precisely what men were then trying 

 to prove. These jumpings from class to class 

 or from era to e.'a, by aid of a few isolated 

 stepping-stones, were what Bacon calls antici- 

 pations, " hasty and premature " but " very 

 effective, because as they are collected from a 

 few instances, and mostly from those which, 

 are of familiar occurrence, they immediately 

 dazzle the intellect and fill the imagination."^ 

 ^0 secure step was taken until the modern 

 paleontologists began to affiliate mutation with 

 mutation and species with species, working his 



s Nov. Org., I., 28. 



