Individuals die, populations disappear, and species become extinct. That is one view of the world. But another view of the world concentrates not so much on presence or absence as upon the numbers of organisms and the degree of constancy of their numbers. These are two very different ways of viewing the behavior of systems and the usefulness of the view depends very much on the properties of the system concerned. If we are examining a particular device designed by the engineer to perform specific tasks under a rather narrow range of predictable external conditions, we are likely to be more concerned with consistent nonvariable performance in which slight departures from the performance goal are immediately counteracted. A quantitative view of the behavior of the system is, therefore, essential. With attention focused upon achieving constancy, the critical events seem to be the amplitude and frequency of oscillations. But if we are dealing with a system profoundly affected by changes external to it, and continually confronted by the unexpected, the constancy of its behavior becomes less important than the persistence of the relationships. Attention shifts, therefore, to the qualitative and to questions of existence or not. Our traditions of analysis in theoretical and empirical ecology have been largely inherited from developments in classical physics and its applied variants. Inevitably, there has been a tendency to emphasize the quantitative rather than the qualitative, for it is important in this tradition to know not just that a quantity is larger than another quantity, but precisely how much larger. It is similarly important, if a quantity fluctuates, to know its amplitude and period of fluctuation. But this orientation may simply reflect an analytic approach developed in one area because it was useful and then transferred to another where it may not be. Our traditional view of natural systems, therefore, might well be less a meaningful reality than a perceptual convenience. There can in some years be more owls and fewer mice and in others, the reverse. Fish populations wax and wane as a natural condition, and insect populations can range over extremes that only logarithmic

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Resilience and Stability of Ecological Systems

Hydrogels in pharmaceutical formulations.

Modeling of drug release from delivery systems based on hydroxypropyl methylcellulose (HPMC).

Novel bioactive materials with different mechanical properties.

Wahrscheinlichkeit-Rechnung. Proc. Tokyo Mathem. Phys. Soc., 2nd Ser., 8: 556-564. Miyasita, K. 1955. Some consideration on the population fluctuation of the rice stem borer. (In Japanese.) Bull. Nat. Inst. Agr. Sci., Japan, C, 5: 99-109. Schwerdtfeger, F. 1935. Uber die Populationsdichte von Bupalus piniarius, Panolis flammea, Dendrolin is pini, Sphinx pinastri und ihren zeitlichen Wechsel. Z. Forst-u. Jagdwesen, 67: 449-482, 513-540. . 1954. Grundsitzliches zur Populationsdynamik der Tiere, insbesondere der Insekten. Allgem. Forst-u. Jagdzeitung. 125: 200-209. Smith, H. S. 1935. The role of biotic factors in the determination of population densities. T. Econ. Ent., 28: 873-898.

Population Ecology of Some Warblers of Northeastern Coniferous Forests

Introduction The diffusion of small molecules in polymeric solids has been a subject in which relatively little interest has been shown by the polymer chemist, in contrast to its counterpart, i.e., the diffusion of macromolecules in dilute solutions. However, during the past ten years there has been a great accumulation of important data on this subject, both experimental and theoretical, and it has become apparent that in many cases diffusion in polymers exhibits features which cannot be expected from classical theories and that such departures are related to the molecular structure characteristic of polymeric solids and gels. Also there have been a number of important contributions to the procedures by which diffusion coefficients of given systems can be determined accurately from experiment. I t is impossible, and apparently beyond the author's ability, to t reat all these recent investigations in the limited space allowed. So, in this article, the author wishes to discuss some selected topics with which he has a relatively greater acquaintance but which he feels are of fundamental importance for understanding the current situation in this field of polymer research. Thus the present paper is a kind of personal note, rather than a balanced review of diverse aspects of recent diffusion studies.

Diffusion in polymer-diluent systems

Titanium metals form direct bonding to bone after alkali and heat treatments.

was obtained in a semi-infinite medium (the medium is initially free from diffusing substance and its surface is maintained at a constant concentration throughout the process). When the parameter X is chosen properly, equation (1) gives a concentration dependence resembling rather closely the types which are often observed in many polymersolvent systems; however, appropriate experimental examples of D-C data, which are represented quantitatively by the above relationship, are not to be found in the existing literature. Accordingly, from the practical point of view, the formal solution given in Part I is still a matter of only theoretical interest. More recently, it was found that there is another case of D(C) which permits a formal solution of the unidimensional diffusion equation subject to the same initial and boundary conditions

The Exact Pattern of a Concentration-Dependent Diffusion in a Semi-infinite Medium, Part II:

The purpose of this study is to evaluate the bone-bonding ability of alkali- and heat-treated titanium alloys. Smoothed-surface rectangular plates of Ti6Al4V, Ti6Al2Nb1Ta, and Ti15Mo5Zr3Al were prepared. The plates were inserted transcortically into the proximal metaphyses of bilateral rabbit tibiae, with alkali- and heat-treated plates inserted on the right side, and untreated plates on the left. The tensile failure loads between the implants and the bones were measured after 8, 16, and 24 weeks by a detaching test. The untreated implants showed almost no bonding even at 16 weeks, and only weak bonding at 24 weeks. In contrast, treated implants showed bonding to bone at all time periods. Histological examination showed that alkali- and heat-treated alloys bonded directly to the bone. Conversely, the untreated implants had an intervening layer of fibrous tissue between the bone and the plate, or only partial direct contact with the bone. This study demonstrates that alkali and heat treatments enhance the bone-bonding strength of these titanium alloys. Although in this study even tentative conditions of the treatments enhance the bonding strength of the titanium alloys, further work is required to determine the optimum conditions for treatment to give the highest bonding strength. These new bioactive titanium alloys are available for weight-bearing and bone-bonding orthopedic devices.

Enhancement of bone-bonding strengths of titanium alloy implants by alkali and heat treatments.

An approximate theory of the relaxation of stress in amorphous linear polymers accompanying sorption of a low molecular weight penetrant is worked out on the assumption that the relaxation time of each Maxwellian relaxation mechanism involved is changed in the presence of penetrant by a factor dependent on penetrant concentration. A method is derived from the theory which permits approximate evaluation of the integral diffusion coefficient D of the penetrant in the polymer from stress-relaxation data on swelling systems. The theory is checked on experimental data for the systems polymethyl acrylate-water and polymethyl acrylate-methanol at 40°C., and it is found that, for both systems studied, the values of D computed from mechanical data agree reasonably with those evaluated directly from usual sorption experiments. The theory described presents only a first attempt to the quantitative interpretation of the phenomenon discussed. Further improvement and modification of it is apparently desirable, in view of some drastic approximations incorporated in its mathematical development.

Hiroshi Fujita

Papers

Diffusion in polymer-diluent systems

Titanium metals form direct bonding to bone after alkali and heat treatments.

The Exact Pattern of a Concentration-Dependent Diffusion in a Semi-infinite Medium, Part II:

Enhancement of bone-bonding strengths of titanium alloy implants by alkali and heat treatments.

Diffusion‐controlled stress relaxation in polymers. III. Stress relaxation in a swelling polymer