24 



NATURE 



[November 2, 191 1 



collection is almost entirely due to Mr. A. N. Leeds, who 

 for the last forty years has closely watched the clay-pits, 

 and has collected most of the specimens, a large series of 

 which is exhibited in the gallery of fossil reptiles in the 

 Natural History Museum at South Kensington. 



Momentum in Evolution. 



In his opening remarks on this subject Prof. Dendy 

 mentioned that Dr. Smith Woodward, in 1909, had directed 

 attention to the fact that many groups of the animal 

 kingdom, in the course of their evolution, have shown a 

 strongly marked tendency to enormous increase in size, 

 often accompanied by the development of grotesque and 

 apparently useless excrescences. Prof. Dendy instanced as 

 analogous phenomena the extraordinary development of the 

 beak and helmet in the hornbills and of the tusks in the 

 babirusa. In these, and in many other cases which could 

 be adduced, either the entire body or some particular organ 

 appears to have acquired some sort of momentum, by virtue 

 of which it has continued to grow far beyond the limits of 

 utility, although perhaps in some cases a new use may 

 have been found which has assisted the species in maintain- 

 ing itself in the struggle for existence. An enormous in- 

 crease in mere bodily size, however, seems in the long run to 

 be always fatal to the race, the place of which will be taken 

 by smaller and more active forms. Prof. Dendy thought 

 there was some ground for believing that a race of animals 

 may acquire a momentum of the kind referred to which 

 may lead ultimately to its destruction, that there is some 

 brake applied to the growth of organs and organisms, but 

 that there are occasions on which the brake may be re- 

 moved, with results which ultimately prove fatal. He 

 pointed out that the growth of different parts of the 

 animal body is controlled by internal secretions, or 

 hormones, the products of various glands. Disease of the 

 pituitary body leads to acromegaly, one of the symptoms 

 of which is great enlargement of certain parts. Prof. 

 Dendy held that there is good reason for believing that, 

 in the absence of certain specific secretions, the growth 

 of the various organs will continue far beyond the normal 

 limits. He saw no reason why this principle should not 

 be extended to the race, and, paradoxical as it might 

 seem, he thought it possible to explain the growth of the 

 organism as a whole and of its various organs beyond the 

 limits of utility as an indirect result of natural selection. 



When a useful organ is first beginning to develop or to 

 take on some new function for which an increase in size 

 will be advantageous, natural selection will favour those 

 individuals in which it grows most rapidly and attains the 

 largest size in the individual lifetime. If growth is 

 normally inhibited by some specific secretion, natural selec- 

 tion will favour those individuals in which the glands 

 which produce this secretion are least developed or least 

 efficient, and, this process being repeated from generation 

 to generation, these glands may ultimately be eliminated, 

 or at any rate cease to produce the particular hormone in 

 question. Moreover, this elimination may take place long 

 before the organ the growth of which is being favoured by 

 natural selection has reached the optimum size. When it 

 has reached this optimum it is certainly desirable that it 

 should grow no larger ; but there is no longer any means 

 by which growth can be checked. The inhibiting hormone 

 can no longer be produced ; the brake has been taken off, 

 and further growth takes place irrespective of utility, until, 

 when the size of the organ, or it may be of the entire 

 organism, becomes incompatible with the well-being of the 

 individual, natural selection again steps in and eliminates 

 the race. Is it not possible that, the normal checks to 

 growth being thus removed along certain lines by the 

 action of natural selection, a definite direction may be 

 given to the course of evolution which the organism will 

 continue to follow to the bitter end, irrespective of natural 

 selection ? 



The Food Supply of Aquatic Animals. 



Dr. W. J. Dakin directed attention to some of the recent 

 work on the nutrition of marine organisms, first citing the 

 work of Putter, who showed that there is more organic 

 carbon present in solution in sea water than in the plankton 

 contained in that water. With the view of showing that 

 aquatic animals do use the food in solution. Dr. Dakin 

 adduced the following observations. A specimen of the 

 NO. 2192, VOL. 88] 



sponge Suberites, of 60 grams weight, requires as fo<»! 

 23 milligrams of carbon per day, to obtain which tl 

 sponge would need to capture 7,400,000,000 Thalassiosi> 

 nana (or an equivalent of other organisms), and won 

 therefore need to filter several thousand times its ov, 

 volume of water per hour; but if the food in solution ; 

 the sea water be also used, a much more rational quaniir 

 of water would suffice to supply the animal's need 

 Extraordinarily large numbers of copepods would be r 

 quired to provide the food of the larger Rhizostomes ; bi 

 it seems impossible that copepods are captured in such lar^ 

 numbers, for copepod remains are so seldom found in t! 

 medusae. Goldfish living in tap water, without solid fo*! 

 were able to exist forty -one days; but with soluble orgar 

 bodies added to the water the fishes lived seventy-eigl 

 days, and the amount of oxygen consumed was found i 

 be in excess of that calculated from the loss in weight • 

 the fish, that is, some o.xygen must have been used i' 

 the oxidation of substances in addition to those stored • 

 the tissues. Many other facts, e.g. that crabs, living '.■ 

 sponges, with only filtered water at their disposal, and th; 

 Daphnia can be kept living and growing in solutions co: 

 taining only dissolved food matter, seem to be in favour 

 Putter's theory. Dr. Dakin believed that, though sol 

 food is necessary, food in solution forms part of the norm, 

 food supply of aquatic organisms. 



Prof. Hartog remarked that there was still much 1 

 discover regarding the nutritive apparatus of the low 

 invertebrates, and that, if Putter's theory be true, tl 

 accessory intestine of some worms and echinoderms, ar 

 the rectal pumping apparatus of worms and Crustacc 

 may serve not only a respiratory, but a nutritive, fun 

 tion. Prof. Herdman suggested that the figures advanc 

 by Putter were not sufficient either to prove or disprox 

 his contentions, and that renewed investigations are nec', - 

 sary. He also pointed out that copepods, which for 

 long time were believed not to take solid food, feed i 

 minute diatoms, which, owing to their very small size, h: 

 until recently not been observed in the gut of the- 

 Crustacea. Prof. Dendy referred to the crabs which li\ 

 in cavities of sponges, and stated that in most of the- 

 cases the crab probably feeds on the sponge, and the latt' 

 regenerates as quickly as it is eaten away. Dr. Gemm 

 stated that one of the difficulties in the way of his accep 

 ing Putter's theory arose from the fact that he was n. 

 aware of any soluble organic food material likely to 1 

 present in sea water which woyld not be immediat< ' 

 attacked and broken down by bacteria. Mr. D. J. Scou; 

 field pointed out the great difficulty of estimating th 

 numbers of organisms, and therefore of solid food, presf: 

 in water, remarking that the rate of increase of small- 

 organisms, e.g. algae, bacteria, is so enormous that thr 

 presence in sufficient numbers in any one catch is r, 

 necessary for the explanation of the feeding of the larg 

 organisms, for the small organisms may very soon becon 

 so abundant as to provide the necessary amount of forii 

 for the larger ones. 



The Systematic Position of the Cyclostomcs. 



Dr. W. W. F. Woodland introduced a discussion on 

 this subject. He described in considerable detail tl 

 innervation, musculature, and cartilages (especially tl 

 lingual) of the head of cyclostomes, and held that recent 

 work showed thar the sub-ocular arch and lingual 

 cartilages could not be regarded as homologous with the 

 palato-pterygo-quadrate bar and glosso-hyal element of 

 gnathostomes. As the musculature of the piston cartilage 

 is innervated by the mandibular nerve, and not by the 

 hypoglossal, modern upholders of the gnathostome ancestry 

 have revived the opinion that the piston cartilage represents 

 the much modified and displaced mandible of gnatho- 

 stomes, the so-called hyoid representing a quadrate element. 

 Dr. Woodland pointed out that the piston cartilage is not 

 paired, that it does not surround the mouth as mandibular 

 elements should,* and held it difficult to believe that it 

 could be a reduced jaw apparatus. The development of 

 the piston musculature and cartilage in the mid-ventral 

 line is inconsistent with the view that they were formerly 

 paired laterally placed mandibular muscles and rami, and 

 the extension of the myotomes laterally in an unbroken 

 series to the extreme end of the head proves that a jaw 



