EVOLUTION 975 



the line. For how long must a character have been absent or latent 

 before its reassertion or reawakening is to be called an atavism? 

 A drone-bee arises from an unfertilised egg, it has a mother and two 

 grandparents, but no father. But it seems rather absurd to call its 

 resemblance to its grandfather atavistic or reversionary. This is a 

 redtictio ad absurdum, for the drone-bee would resemble its father 

 if it had one ! The case may serve to show that it is undesirable to 

 use the term "atavism" imless the throw-back is to an ancestor 

 more than two generations antecedent. 



The exact study of atavistic phenomena must have regard to 

 characters which can be definitely measured and registered, and 

 only when this study has reached secure results will it be possible to 

 discuss with precision what may be called psychical atavisms, that 

 is to say, reawakenings, often more fitly termed recrudescences, 

 of ancestral traits which have lain latent, it may be, for generations. 

 The garden of a shepherd's cottage which was swallowed up in a 

 deer forest lost all trace of its previous cultivation, and became a 

 weed-ground. After many years, under more humane conditions, it 

 was re-delved, and there sprang up many different kinds of old- 

 fashioned flowers whose seeds had lain dormant for several genera- 

 tions. So may ancient flowers and weeds now and again reappear 

 out of latency in that garden which we call our inheritance. They 

 illustrate atavism or reversion. 



THE PULSE OF EVOLUTION 



The very suggestive phrase "pulse of life" has been used almost 

 technically by Lull, the distinguished palaeontologist of Yale, to 

 denote the occurrence of "big lifts" in Organic Evolution, followed 

 by periods which could not be called eventful. And besides the 

 big hfts there have been downward plunges, when great races 

 of animals passed into extinction. There have been crises in organic 

 evolution, such as the great advance of the fore-brain in the 

 monkeyish animals that became arboreal in the Eocene Period, or 

 the emergence of birds from Dinosaurian ancestors in the Jurassic 

 Period or earUer. No doubt it is difficult to be quite sure when the 

 various great Vertebrate initiatives were made: and as for Inverte- 

 brates we can hardly tell at all, since there are representative 

 fossils of most of the chief groups of backboneless animals in Cam- 

 brian rocks. But it will be admitted by aU that in certain periods 

 there was a very marked advance of certain forms of life. The 

 problem is how far these upward movements express some pro- 

 pitious conditions of climate and the rest of the physical environ- 

 ment, and how far they may also express an internal momentum in 

 evolution. 



