C. — GEOLOGY. 



77 



been explained by the assumption that both creatures are descended, 

 longo intervallo no doubt, from a common stock, and that the flesh or 

 the germ of that stock had the internal impulse to produce this kind of 

 eye some day when conditions should be favourable. It is not ex- 

 plained why many other eyed animals, which must also have descended 

 from this remote stock, have developed eyes of a different kind. Never- 

 theless I commend this hypothesis of Professor Bergson to the advo- 

 cates of predisposition. To my mind it only shows that a philosopher 

 may achieve distinction by a theory of evolution without a secure know- 

 ledge of biology. 



When the same stock follows two quite different paths to the same 

 goal, it is impossible to speak of a predetermined course. In the Wen- 

 lock beds is a crinoid whose stalk has become flattened and coiled, and 

 the cirri or tendrils of the stalk are no longer set by fives all round it, 

 but are reduced to two rows, one along each side. In one species these 

 cirri ai-e spaced at iri'egular intervals along the two sides, but as the 

 animal grows there is a tendency for them to become more closely set. In 

 another species, in various respects more developed, the cirri are set quite 

 close together, and the tightly coiled stalk looks like a ribbed ammonite. 

 Closer inspection shows that this species includes two distinct forms. 

 In one each segment of the stalk bears but a single cirrus, first on the 

 right, then on the left ; but the segments taper off to the opposite side so 

 that the cirri are brought close together. In the other form two cirri 

 are borne by a single segment, but the next segment bears no cirri. 

 These intervening segments taper to each side, so that here also the 

 cirri are brought close together. Thus the same appearance and the 

 same physiological effect are produced in two distinct ways. Had one 

 of these never existed, the evolution of this curious stem would have 

 offered as good an argument for oxthogenesis as many that have been 

 advanced. So much for similarity ! 



The argument for orthogenesis based on a race-history that marches 

 to inevitable destruction, heedless of environmental factors, has always 

 seemed to me incontrovertible, and so long as my knowledge of 

 palaeontology was derived mainly from books I accepted this premiss 

 as well founded. Greater familiarity with particular groups has led 

 me to doubt whether such cases really occur, for more intensive study 

 generally shows that characters at first regarded as indifferent or 

 detrimental may have been adapted to some factor in the environment 

 or some peculiar mode of life. 



Professor Duerden's interesting and valuable studies of the ostrich 

 (1919, 1920, Journ. Genetics) lead him to the opinion that retrogressive 

 changes in that bird are destined to continue, and ' we may look for- 

 ward,' he says, ' to the sad spectacle of a wingless, legless, and feather- 

 less ostrich if extinction does not supervene. ' Were this so we might 

 at least console ourselves with the thought that the process is a very 

 slow one, for Dr. Andrews tells me that the feet and other known 

 bones of a Pliocene ostrich are scarcely distinguishable from those of 

 the present species. But, after careful examination of Dr. Duerden's 

 arguments, I see no ground for supposing that the changes are other 

 than adaptive. Similar changes occur in other birds of other stocks 



