April 14, 1922] 



SCIENCE 



399 



oceanic evolution than my attitude toward bio- 

 logical evolution when, twenty years ago I 

 supposed, as Professor Osbom seems still to 

 suppose, that search for causes of this latter 

 evolution is the supreme goal of biological 

 study. 



But I am mindful that there is a reason why 

 biologists have been goaded to strain them- 

 selves more in search of originating causes in 

 their domain than have other scientists in 

 search of such causes in their domains. That 

 reason is the historic circumstance that these 

 other scientists have long since been relieved 

 of danger from the germ of supernatural 

 causation in their domains, while this genn 

 still lingers in the biological domain. 



The way by which biology may escape limbo 

 in this matter, Batoson, along with a consider- 

 able number of naturalistic biologists, is ap- 

 parently beginning to see. "Meanwhile," he 

 says, "our faith in evolution stands unshaken." 



What is the lesson, practical and theoretical, 

 implied in such a declaration? What it is 

 for Bateson of eoui'se I do not know. For my- 

 self it is this: Le{ us stop trying to convince 

 ourselves and others that we have discovered, 

 or in a few minutes loill discover, the causes 

 of evolution, and devote our efforts to per- 

 ceiving for ourselves and convincing others of 

 the naturalness, through-and-through , of evo- 

 lution. In other words, let us bestoiv much 

 more time and energy upon the grounds of 

 our faith in evolution as one of nature's grand- 

 est processes, than upon searching after, and 

 speculating about, the causes of evolution. 



Wm. E. Ritter 

 University of California, 



ScRipps Institution for 

 Biological Research, 

 March 4, 1922 



FURTHER CONSIDERATION OF THE SIZE OF 



VEIN-ISLETS OF LEAVES AS AN 



AGE-DETERMINANT 



In a recent paper'- regarding vein-islet 



measurements as a means of determining the 



1 Ensign, M. E., Area of vein-islets in leaves of 

 certain plants as an age determinant, Jour. Bot., 

 S: 433, 1921. 



age of the woody perennial upon which the 

 leaves are borne, the use of fresh leaves under 

 low mag-nifieation was criticised. The basis 

 of the criticism was that different thicknesses 

 of chlorophyll would affect the number of vein- 

 lets visible and thus affect the apparent size 

 of the vein-islets. This of course, is true. The 

 fact that it is true constitutes one of the im- 

 portant advantages of the method criticised 

 and is an equally important objection to the 

 sole use of the suggested method. 



In my original paper it was pointed out that 

 the palisade cells decreased in size with age 

 as do all of tlie other kinds of cells in the leaf 

 with the exception of the cells of the veinlets 

 which increase somewhat in size. With in- 

 creasing age both the lessening thickness of the 

 chlorophyll-containing cells and tiie increas- 

 ing size of the conducting cells will render 

 the veinlets more conspicuous. The actual in- 

 crease in the amount of conducting tissue in 

 the leaf is emphasized by increased visibility. 

 The use of fresh material under low magnifica- 

 tion gives a morphological summation which 

 the suggested method quite lacks and there- 

 fore I adopted it after a trial of both. In this 

 case the method adapted to field use is the 

 more precise, as an age-determiner. 



As is well known, the venation of the leaf of 

 any given species is affected by external agents. 

 Different species respond to these factors in 

 different ways. Since the size of the vein- 

 islets is affected by these factors, a successful 

 use of this method of age determination re- 

 Cjuires sufficient familiarity with the responses 

 of the species used, to enable one to eliminate 

 the differences not due to age. As soon as 

 this is done the relation of the size of the vein- 

 islets to age is clear. 



Since the discovery that the "protoplasm" 

 of plants was fundamentally the same as the 

 "sarcode" of animals, the progress of physiol- 

 ogy has been steadily toward a demonstration 

 that in the essentials of composition and re- 

 sponse the two are essentially alike. Any 

 theory of senility which can not be applied to 

 pilant conditions is not a fundamental theory 

 and can l>e disi'egarded. It is equally true 

 that any characteristic so strongly marked as 



