Saltatory Evolution 23 



medusoid from a free-swimming animal to a mere brood-sac (gono- 

 pliore) is not sudden and saltatory, but occurs by imperceptible 

 modifications throughout hundreds of years, as we can learn from 

 the numerous stages of the process of degeueratiou persisting at the 

 same time in different species. 



If, then, the degeneration to a simple brood-sac takes place only 

 by very slow transitions, each stage of Avhich may last for centuries, 

 how could the much more complex ascending evolution possibly have 

 taken place by sudden leaps ? I regard this argument as capable of 

 further extension, for wherever in nature we come upon degeneration, 

 it is taking place by minute steps and with a slowness that makes it 

 not directly perceptible, and I believe that this in itself justifies us 

 in concluding that the same must he true of ascending evolution. 

 But in the latter case the goal can seldom be distinctly recognised 

 while in cases of degeneration the starting-point of the process can 

 often be inferred, because several nearly related species may repre- 

 sent different stages. 



In recent years Bateson in particular has championed the idea of 

 saltatory, or so-called discontinuous evolution, and has collected a 

 number of cases in which more or less marked variations have 

 suddenly appeared. These are taken for the most part from among 

 domesticated animals which have been bred and crossed for a long 

 time, and it is hardly to be wondered at that their much mixed and 

 much influenced germ-plasm should, under certain conditions, give 

 rise to remarkable phenomena, often indeed producing forms which 

 are strongly suggestive of monstrosities, and which would undoubtedly 

 not survive in free nature, unprotected by man. I should regard such 

 cases as due to an intensified germinal selection — though this is to 

 anticipate a little — and from this point of view it cannot be denied 

 that they have a special interest. But they seem to me to have no 

 significance as far as the transformation of species is concerned, if 

 only because of the extreme rarity of their occurrence. 



There are, however, many variations which have appeared in a 

 sudden and saltatory manner, and some of these Darwin pointed out 

 and discussed in detail : the copper beech, the weeping trees, the oak 

 with " fern-like leaves," certain garden-flowers, etc. But none of them 

 Iiave persisted in free nature, or evolved into permanent types. 



On tlie other hand, wherever enduring types have arisen, we find 

 traces of a gradual origin by successive stages, even if, at first sight, 

 tlieir origin may appear to have been sudden. This is the case with 

 8caso)iat diniorphisnt, the first known cases of which exhibited 

 marked ditterences between the two generations, the winter and tlie 

 summer brood. Take for instance the much discussed and studied form 

 Vanessa (Araschnia) levana-prorsa. Hero the differences between 



