November 24, 1922] 



SCIENCE 



591 



explanation of adaptation, if adequate, destroys 

 completely the very keystone of the arch of any 

 theistic philosophy. Nothing could undermine 

 more completely the prestige of a theistic 

 agency than to prove that it is unnecessary — 

 than to show, in short, that the supposed re- 

 sults of its infinite wisdom and omniscience not 

 only would have occurred, but actually did hap- 

 pen as a result purely of natural, mechanical 

 causes without any external, supernatural inter- 

 vention. 



The question, however, is : did the manifold 

 adiaptations which we see in living nature in 

 actual fact arise through the operation of the 

 processes of trial and error and natural selec- 

 tion? A final answer to this question seems 

 to me impossible in the present state of knowl- 

 edge. In the eighties and nineties the answer 

 would have been, among biologists if not among 

 philosophers, almost unanimously affirmative. 

 Today the case seems much more doubtful. 

 Formally it is possible to explain many par- 

 ticular adaptations by natural selection. Some 

 it appears impossible to explain in this way, 

 even formally. What wants intensive investi- 

 gation is the whole biology, from every con- 

 ceivable angle, of particular adaptations. No 

 more important pix)blem exists. And its diffi- 

 eulty should act as a stimulus rather than a 

 deterrent to its study. To solve it, or indeed 

 to contribute significantly (to its solution, will 

 require a different point of view and a differ- 

 ent method from that of present-day genetics. 



It may seem a little ungracious to suggest, 

 in view of the 'brilliant results of genetic work 

 which I have already mentioned, and which I 

 yield to no one in admii'ation of, that the 

 present dominant mode of research in genetics 

 can give ixs only an incomplete and, philo- 

 sophically considered, somewhat superficial 

 knowledge of heredity, but I am unable to 

 convince myself that such is not the fact. My 

 views on this point have not changed since I 

 discussed it in detail some seven years ago. I 

 then said*' : — 



Mendelism finds its limitations, just as did the 



6 Pearl, E. Modes of Research in Genetics. 

 New York (Maemillan), 1915. 



biometrie methods in the fact that from the logi- 

 cal standpoint it is essentially a statdstieal meth- 

 od which studies only the laws of distribution of 

 things given or assumed. It examines only the 

 distribution of hereditary specificities, and not 

 at all, directly, their origin or determination. The 

 former aim cannot be tlie goal of genetic science. 

 A method which can travel only so far cannot 

 hope to say the last word in the discussion of the 

 problem of heredity. As a mode of research the 

 Mendelian method of analyzing the progeny dis- 

 tributions rather than tlie ancestral will always 

 be used. It was indeed one of the most brilliant 

 methodological discoveries in the history of sci- 

 ence. But it has limitaitions in the direction of 

 what it can accomplish per se in elucidating the 

 problem of heredity. 



It is altogether usual in current discussions of 

 variation and heredity to neglect completely 

 everything which comes between the two end terms 

 of the ontogenetic series, the germ cell on the one 

 hand and the adult soma on the other. But clearly 

 what goes between is a most essential part of 

 heredity itself. It is astonishing how little has 

 been done on these extremely obvious problems. 



Two of the four general methods which have 

 been employed in the investigations of the prob- 

 lem of heredity have been seen to be essentially 

 statistical, and two essentially biological. The 

 statistical methods — ^the biometrie and the Men- 

 delian — differ fundamentally only in that the 

 former investigates primarily the ancestry and 

 the latter primarily the progeny. Logically ex- 

 actly the same distinction was found between the 

 two purely biological methods — the eytologieal 

 and the embryological. The former studies the 

 ancestry of the germ cell (gametogenesis), the 

 latter the progeny of the germ cell (somato- 

 genesis). 



All of these methods are valuable, and each 

 has contributed to our present knowledge of 

 heredity. No one of the methods alone can, how- 

 ever, solve the problem. They all have at least 

 one fundamental limitation in common. This is 

 that they offer no means of directly getting at 

 any definite information regarding the origin, 

 cause, or real nature of that specificit}' of living 

 material which is the very foundation of the 

 phenomenon of heredity. The distribution of 

 hereditary specificities, their putative morphologi- 

 cal "bearers," and many other thiugs about 

 them have been studied more or less exhaustively. 

 The things themselves have been speculated about, 

 but not investigated to any but the slightest ex- 

 tent. 



