April s, 1888] 



NATURE 



541 



iodine. But in arranging the elements in vertical series 

 we have often great difficulty in determining which are 

 true homologues : we have no difficulty in grouping 

 the alkali metals, the halogens, or sulphur, selenium 

 and tellurium, but how are we to place copper, 

 silver and gold, for example ? Are we justified in 

 regarding them as true homologues, and in inserting them 

 as intermediate terms in the group of the alkali metals ? 

 Ought we not rather to look upon them as but rseudo- 

 homologues, and ought we not to place them apart from 

 the alkali metals, and apart even from each other in 

 vertical succession ? This would lead us, instead of 

 classifying the elements in linear vertical series, to 

 arrange them in pyramidal groups, of which the 

 elements of lowest weight form the summits. In fact, 

 there is no justification whatever for the conclusion 

 that the elements belong to only eight families ; the 

 most illiberal treatment leads us to recognize at least 

 twelve, and there is no reason to accept this as the limit. 

 We can thus foresee the possible existence of a far 

 larger number of elements than is at present known, 

 differing probably from each other to a marked ex- 

 tent both in atomic weight and properties. But 

 even then the limit is not reached. Those who have 

 classified the elements according to the periodic sys- 

 tem, after all — consciously or unconsciously — have but 

 followed the practice adopted in classifying carbon com- 

 pounds ; and if we consider the results arrived at by the 

 study of hydrocarbons, and apply the conclusions to the 

 elements, there appears to be no difficulty in finding place 

 for a far larger number of meta-elements than even Kriiss 

 and Nilson would require to accommodate their host of new 

 claimants for elemental rank. If we arrange homologous 

 hydrocarbons side by side in the order of molecular 

 weight, a scheme corresponding to that devised for the 

 elements will result ; but, if molecular weight only be 

 considered, the existence of isomeric hydrocarbons 

 escapes notice : if, however, isomers are included, each 

 simple vertical group at once assumes a pyramidal form. 

 In like manner, if the possible existence of isomeric ele- 

 ments be granted, the periodic scheme would admit with- 

 out difficulty of the existence of a still larger number of 

 elements even than was above indicated. 



Nickel and cobalt have often been supposed to be 

 isomeric elements. According to the most recent deter- 

 minations of their atomic weights, however, cobalt has a 

 higher weight (5874) than nickel (58'56) ; but this result 

 is discredited by the fact that cobalt is usually placed 

 before nickel in the periodic scheme, and should therefore 

 have the lower weight, unless the two elements are 

 isomeric. 



Whether among the meta-elements of the rare earths 

 there are not numerous cases of isomerism, remains for 

 the future to determine. Unless, however, some new 

 mode of discriminating other than that involved in deter- 

 mining the atomic weight be introduced, the problem is 

 one which appears beyond our present powers, as experi- 

 mental error cannot be entirely eliminated. But it is 

 perhaps of all the problems in chemistry the most 

 important to solve, on account of its bearing on the higher 

 problem whether the elements are simple or compound 

 substances. So many converging lines of evidence now 

 render it probable that the elements are compounds that 

 the discovery of isomeric elements would probably suffice 

 to carry conviction to the minds of all who are open to 

 argument on this question. H. E. A. 



THE DURATION OF LIFE} 



TOHANNES MOLLER, the celebrated German 

 J zoologist, said : " All organic beings are transitory ; 

 life passes from individual to individual with the appear- 



' " Ueber die Dauer des Lebens." Von Dr. August Weismann. (Jena, 

 1882.) 



ance of immortality, but the individuals themselves 

 perish." This proposition is. perhaps not so true as it 

 seems to be. Nevertheless, it is certainly true that life 

 has its natural limits, at least in all those animals and 

 plants that ordinarily come under the notice of the layman. 

 But the duration of life is very different in different 

 animals, and it would be interesting to know the reason 

 of this. Differences in length of life have been thought to 

 depend on differences in structure and composition. 

 Obviously the size of an animal will fix a certain minimum 

 of time required for growth : owing to the relation between 

 increase of bulk and increase of absorbent surface, pointed 

 out by Leuckhart and Spencer, a larger animal will 

 require a longer time to secure the surplus of nutriment 

 required for reproduction. The degree of structural com- 

 plication will also fix a minimum time : the activity of 

 the vital processes, the rate of metabolism, — because it 

 influences the time at which reproductive power, the 

 goal of individual life, is reached — will influence the 

 total duration of life. IBut these inner conditions do not 

 fix the duration of life. Birds, whose vital processes are 

 so rapid, may far surpass in age the sluggish Amphibia. 

 Among! ants, the males, females, and workers are prac- 

 tically identical in size, complication of structure, or rate 

 of metabolism ; yet the females and workers live several 

 years, the males only a few weeks. 



We must seek in the environment for the forces finally 

 determining the duration of life. We find the length of 

 life to be in each case an adaptation arranged by natural 

 selection in the interests of the species. So soon as an 

 individual has produced young enough to fill up the gaps 

 caused by death, it ceases to be of use for the species. 

 Where fostering of the brood obtains — be it uterine or 

 post-uterine — we expect and find a longer duration. 



The apparently accidental causes of death remove far 

 more individuals than natural death. The longer an 

 individual lives the more chances of accident does it 

 undergo ; and so selection, acting in the interests of the 

 species, rather than prolonging the life, hurries on the time 

 of reproduction. At first, it seems impossible that the 

 great age reached by many birds (Raptores may survive 

 their century) is the shortest possible. But the enemies 

 of the eggs and of the young of birds are very numerous. 

 The death-rate is enormously greater than in the case 

 of mammalian embryos developing within the parent. 

 Adaptation to rapid flight precludes great fertility. Bad 

 fliers like the Phasianidce lay many more eggs in a season 

 and live through far fewer seasons. 



The adaptation is very clear in the case of the larval 

 life of insects. The larva; of bees and of many ichneumons 

 placed in the midst of an abundant food supply become 

 pupae in a few days. The larval stage of predacious larvae 

 which have to waste time and energy in securing their 

 prey, and of vegetable-feeding larvae, on account of the 

 less nutritious nature of their food, lasts very much longer. 

 The usually short life of the imago bears no relation to 

 the length or shortness of the larval life, but is directly 

 adapted to its own purposes. In the simplest case, where 

 copulation takes place as soon as the wings are dried, and 

 where the eggs are deposited rapidly and carelessly, the 

 whole adult life lasts but a few hours. Where the mate 

 has to be sought, or the eggs deposited in special conditions, 

 or where active habits preclude simultaneous maturation 

 of eggs, the duration of life is prolonged in correspondence 

 with the special requirements. Adult insects are perhaps 

 the most hunted of animals, and in them is found the 

 extreme case of adaptive shortening. 



The inner changes on which natural death depends are 

 not very clear. They can hardly depend on cell destruc- 

 tion ; for it is upon that that the processes of life are based. 

 More probably they depend on a failure to produce new 

 generations of cells to replace the cells broken down in 

 the vital processes. 



The occurrence of death at all is a provision to secure 



