464 SUMMARY AND CONCLUSION. 



of life or environment, into those for example, that live in fresh or salt water, or 

 in the air, or on the land, etc. 



If variation, from whatever cause, is invariably minute and of equal specific 

 value, then a perfect record of the past, that is, the genealogical tree, should show 

 a continuous system of branches composed of a uniformly graded series of animals. 



But while it may be possible to arrange the members of certain small sub- 

 divisions of the animal kingdom into minutely graded linear series, it is usually 

 exceedingly difficult to determine which end of the series is the base and which 

 is the apex, because a considerable gap usually exists between the offshoot and the 

 presumably parent stock. 



The speculative zoologist often attempts to fill this gap with hypothetical 

 forms having the desired intermediate characters, on the assumption that con- 

 necting adult forms, as numerous and finely graded as on any of the modern 

 terminal branches, once existed, but are now either extinct, or unknown, or 

 both extinct and unknown. Some morphologists assume, for example, that the 

 vertebrate stock had its origin in such forms as the annelids, echinoderms, tuni- 

 cates, or enteropneusta, and that the enormous series of animals necessary to fill 

 the gap between them and the vertebrates are now extinct and will remain for- 

 ever unknown. 



But there is no evidence to show that the true genealogical tree is, or should 

 be, a minutely and uniformly graded series of animal forms. On the contrary, 

 we have shown that the internal creative factors are of very unequal value and 

 that they are exceedingly variable at different times, giving rise to well defined 

 periods in phylogeny during which many large and important changes of form take 

 place. Hence while some of the apparent gaps in our imperfect genealogies may 

 be filled by the discovery of new forms, it is evident that the ideal phylogenetic 

 tree of the animal kingdom must be in reality a loosely articulated system, con- 

 sisting of slender, interrupted, or vanishing basal stems, expanding into top- 

 heavy branches. In other words, it was minutely graded in certain places only, 

 notably on the older terminal branches. There the differences may well be either 

 the minute so-called continuous variations of Darwin, or the sharper, discon- 

 tinuous mutations of De Vries. 



But the larger natural subdivisions of the animal kingdom, in some cases at 

 least, appear to be separated by real gaps, much larger than any known mutations, 

 and that are not due to defective records. They represent periods, varying in 

 intensity and in duration, of rapid transformation in definite, predetermined di- 

 rections, followed by periods of slow development along more varied, but less 

 revolutionary lines. These gaps can never be filled by the discovery of new 

 forms, for in reality they represent changes of pace in evolution, or periods of 

 greatly accelerated evolution, which in perspective, and by comparison with 

 other periods, appear as gaps. 



A phyletic metamorphosis of this character I have named a methallosis.^ 



' To rush after, to leap from one ship to another. 



