Lives of Flies and Men

The Rateof Living. By Raymond Pearl'. (University of London HERE is Professor Pearl again with his nose -to the trail of

the fundamental lavis of life. " •

He concludes, among other things; that the harder one lives the quicker one dies. The methods' by which he arrives at this conclusion are interesting. Seeds from the same eanteloup melon—which should therefore 'be of approximately the same genetical (hereditary}- constitution—were - grown without food or light. Deprived of all " ineome " from environment, they grew and lived but a shoit time;, each depending only ^ upon ita - inherent vitality: Those that most quickly sprouted died the soonest. The " catch " here is that even between seeds from the same melon there are genetical differences, sufficient to cause the difference is metabolism. The very fact that they reacted differently

to the same environment suggests simply that some were more precocious than others—which is a very dissimilar conclusion from Professor Pearl's.

From these seeds he makes, as is rather his habit, a colossal jump to a far distant organism—man Those following exceptionally laborious trades have (according to the English occupational mortality tables) an early age of death compared with those doing easier jobs.

But, Unfortunately, nothing is known of the genetical constitution of those human beings. Trades are, to a con- siderable degree, selective ; and one may reasonably infer that no man would take on an exceptionally laborious job if he could get something better. If he cannot, he lacks some quality ; and that quality is likely to be, in the wide sense, vitality.

Again, while work indeed may kill, it is often found

associated with the two closely correlated qualities, longevity and high fertility—which rather suggests_ that those, with enough inherent vitality can " go the pace," within wide

limits, and still outlive the rest of us. - Professor Pearl's other conclusions are (1) that vitality is a strictly hereditary character; but (2) that its expression in the form of longevity is only so in a limited degree.

He experimented upon Drosophila Melanogaster. This little banana-fly has thrown much light on the laws of life.

Never in biology have the structure, habits, breeding, life and death, and general private life of any organism been so investigated. If only for that reason (as well as for many others) it is, as Professor Pearl points out, ideal experimental material. Too many hopeful experiments have been utterly ruined by the fact that the organisms 'Studied were, in effect, completely unknown quantities. The intensive research on Drosophila, too, has cleared up the puzzles of biologists dealing with many other living things.

Professor Pearl has already used it, along with other organisms, to demonstrate a certain rhythm or cycle of population growth, the significance of which to us lies in its parallel among some, if not all, human communities, and in its implication that the growth of a population is in some way automatically regulated by its own density.

Two types of Drosophila were used in the present experiment —the normal wild type and a vestigial-winged variety which

probably could not live in natural surroundings. Repre-

sentative samples were reared (a) in conditions of normal feeding, and (b) in complete starvation like the melon seeds. Let us take the wild type first. Death, naturally, came far swifter in (b) than in (a), but the life curve remained practically the same—translate days of life, roughly, into hours of life, and there is the same picture of the coming of death in both good and bad circumstances. The proportions and relative length of life of long and short-lived individuals were not altered by the change of environment. Further— this is most significant—translate days into years, and the life curve of England and Wales is almost identical with that of the flies in both good and bad circumstances. The charts are striking. Females, by the way, in both show greater longevity than males.

Populations of men and of normal flies both contain the same proportions of long and short-lived individuals. That inherited vitality which determines length of life appears to be dealt out to us in precisely the same way as to Drosophila, and to be as uninfluenced, relatively, by environmental factors.

But the life-curves of the vestigial-winged type appear at first to upset this conclusion. Reared in normal conditions they show, as one would expect from an abnormal type, a much shorter absolute duration of life and an abnormal curve —flattened almost to a diagcinal line, the individuals dying off in nearly equal numbers at successive ages. But for those reared in starvation the absolute duration of life equals that of the wild type in the same conditions, and the curve closely approximates to that of human beings and of the wild type in both sets of conditions.

From this one can deduce, with Professor Pearl—though not adopting his rather unfortunate vocabulary—that indi- viduals and races differ in inherited vitality (as, probably, in all qualities) ; but that the expression of that vitality is to some extent dependent upon a suitable environment. In short, as he does not say, the true criterion of individual differences is maximum, not minimum, potentiality. That can only come to expression in the maximum of opportunity.

The essential superiority of the wild over the vestigial type was its greater capacity to take full advantage of a normal environment. Restrict the environment, and that capacity cannot be utilized to the same degree. Carry the experiment to the extreme and deprive the flies of air, and one will die as quickly as another.

Working on Mendelian lines, Professor Pearl devised a very shrewd further test of the inherent quality of longevity.

He crossed the wild and vestigials. The hybrids were longer- lived than even their wild parents—a natural, but certainly not inevitable (as Professor Pearl implies) consequence of

hybridization. _ Further crossing of the hybrids resulted in the next and folleiving generations in the appearance of throwbacks," wild and- -vestigials,- in the normal simple

Mendelian ratio of one each to two hybrids. These " throw. 13acks," when reared in identical conditions, showed precisely the same longevity qualities as their respective ancestors of similar type, wild or vestigial. Vestigial wings in themselves, it should be added, did not cause shortness of life, for the duration of life of the wild type was not materially altered by the clipping off of their wings. Natural winglessness and shortness of life both appear to be signs of the same inherent lack of vitality. But the vigorous wild type seem to be practically unaffected by so serious a mutilation as the Surgical removal of their wings.

In the reviewer's opinion Professor Pearl's work is more than significant. By 'his imaginative ,yet careful use of admirable statistical and laboratory Methods and research

technique, he does seem to haire burrowed down to some fundamental laws of life affecting flies, men, and probably other organisms. On the other hand, his deductions, as in the case of the melon seeds, are not always impeccable, and he leaps too easily the gulf between men and flies. The gulf could and should be bridged by experimenting in the same way with some animal of nearer kin to man—rats or mice would do excellently. The confirmation then would be extremely strong.

The effect of density of population upon longevity was also tested ; and one remarkable circumstance is worth quoting :—

" . . . one particular environmental factor, density of population, is capable of converting the normal life-curve characteristic of the wild type flies into the diagonal type of life-curve, with its short absolute duration of life characteristic of vestigial flies at optimal densities of population."

Too great density damps down the wild type, putting them

on a level with the vestigials, more than does too slight density.

Remembering still the risk of drawing paxallels between flies and men, this does suggest that a feeling of lack of elbow- room at least lessens the advantages of the vigorous over the weakly. There certainly are indications from other sources that quantity of population reacts on quality, and that too great density has bad effects. Experiments should now be devised to ascertain how the two types of flies breed, as well as die, in these various circumstances.

This is not a book for light reading, and the lay reader will have to take the mathematics for granted. But he is recommended to persevere, not Only on account of the signi- ficance of this work, but for the sheer interest he will derive from learning how biological research is done.

ELDON MOORE.