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（c） THE TROPIC REACTIONS OF CERTAIN TISSUE…CELLS AND THE MORPHOGENETIC EFFECTS OF THESE REACTIONS。
Since plant…cells show heliotropic reactions identical with those of animals； it is not surprising that certain tissue…cells also show reactions which belong to the class of tropisms。 These reactions of tissue…cells are of special interest by reason of their bearing upon the inheritance of morphological characters。 An example of this is found in the tiger…like marking of the yolk…sac of the embryo of Fundulus and in the marking of the young fish itself。 The writer found that the former is entirely； and the latter at least in part； due to the creeping of the chromatophores upon the blood…vessels。 The chromatophores are at first scattered irregularly over the yolk…sac and show their characteristic ramifications。 There is at that time no definite relation between blood…vessels and chromatophores。 As soon as a ramification of a chromatophore comes in contact with a blood… vessel the whole mass of the chromatophore creeps gradually on the blood… vessel and forms a complete sheath around the vessel； until finally all the chromatophores form a sheath around the vessels and no more pigment cells are found in the meshes between the vessels。 Nobody who has not actually watched the process of the creeping of the chromatophores upon the blood… vessels would anticipate that the tiger…like colouration of the yolk…sac in the later stages of the development was brought about in this way。 Similar facts can be observed in regard to the first marking of the embryo itself。 The writer is inclined to believe that we are here dealing with a case of chemotropism； and that the oxygen of the blood may be the cause of the spreading of the chromatophores around the blood…vessels。 Certain observations seem to indicate the possibility that in the adult the chromatophores have； in some forms at least； a more rigid structure and are prevented from acting in the way indicated。 It seems to the writer that such observations as those made on Fundulus might simplify the problem of the hereditary transmission of certain markings。
Driesch has found that a tropism underlies the arrangement of the skeleton in the pluteus larvae of the sea…urchin。 The position of this skeleton is predetermined by the arrangement of the mesenchyme cells； and Driesch has shown that these cells migrate actively to the place of their destination； possibly led there under the influence of certain chemical substances。 When Driesch scattered these cells mechanically before their migration； they nevertheless reached their destination。
In the developing eggs of insects the nuclei； together with some cytoplasm； migrate to the periphery of the egg。 Herbst pointed out that this might be a case of chemotropism； caused by the oxygen surrounding the egg。 The writer has expressed the opinion that the formation of the blastula may be caused generally by a tropic reaction of the blastomeres； the latter being forced by an outside influence to creep to the surface of the egg。
These examples may suffice to indicate that the arrangement of definite groups of cells and the morphological effects resulting therefrom may be determined by forces lying outside the cells。 Since these forces are ubiquitous and constant it appears as if we were dealing exclusively with the influence of a gamete； while in reality all that it is necessary for the gamete to transmit is a certain form of irritability。
（d） FACTORS WHICH DETERMINE PLACE AND TIME FOR THE DEPOSITION OF EGGS。
For the preservation of species the instinct of animals to lay their eggs in places in which the young larvae find their food and can develop is of paramount importance。 A simple example of this instinct is the fact that the common fly lays its eggs on putrid material which serves as food for the young larvae。 When a piece of meat and of fat of the same animal are placed side by side； the fly will deposit its eggs upon the meat on which the larvae can grow； and not upon the fat； on which they would starve。 Here we are dealing with the effect of a volatile nitrogenous substance which reflexly causes the peristaltic motions for the laying of the egg in the female fly。
Kammerer has investigated the conditions for the laying of eggs in two forms of salamanders； e。g。 Salamandra atra and S。 maculosa。 In both forms the eggs are fertilised in the body and begin to develop in the uterus。 Since there is room only for a few larvae in the uterus； a large number of eggs perish and this number is the greater the longer the period of gestation。 It thus happens that when the animals retain their eggs a long time； very few young ones are born； and these are in a rather advanced stage of development； owing to the long time which elapsed since they were fertilised。 When the animal lays its eggs comparatively soon after copulation； many eggs （from 12 to 72） are produced and the larvae are of course in an early stage of development。 In the early stage the larvae possess gills and can therefore live in water； while in later stages they have no gills and breathe through their lungs。 Kammerer showed that both forms of Salamandra can be induced to lay their eggs early or late； according to the physical conditions surrounding them。 If they are kept in water or in proximity to water and in a moist atmosphere they have a tendency to lay their eggs earlier and a comparatively high temperature enhances the tendency to shorten the period of gestation。 If the salamanders are kept in comparative dryness they show a tendency to lay their eggs rather late and a low temperature enhances this tendency。
Since Salamandra atra is found in rather dry alpine regions with a relatively low temperature and Salamandra maculosa in lower regions with plenty of water and a higher temperature； the fact that S。 atra bears young which are already developed and beyond the stage of aquatic life； while S。 maculosa bears young ones in an earlier stage； has been termed adaptation。 Kammerer's experiments； however； show that we are dealing with the direct effects of definite outside forces。 While we may speak of adaptation when all or some of the variables which determine a reaction are unknown； it is obviously in the interest of further scientific progress to connect cause and effect directly whenever our knowledge allows us to do so。
VII。 CONCLUDING REMARKS。
The discovery of De Vries； that new species may arise by mutation and the wide if not universal applicability of Mendel's Law to phenomena of heredity； as shown especially by Bateson and his pupils； must； for the time being； if not permanently； serve as a basis for theories of evolution。 These discoveries place before the experimental biologist the definite task of producing mutations by physico…chemical means。 It is true that certain authors claim to have succeeded in this； but the writer wishes to apologise to these authors for his inability to convince himself of the validity of their claims at the present moment。 He thinks that only continued breeding of these apparent mutants through several generations can afford convincing evidence that we are here dealing with mutants rather than with merely pathological variations。
What was said in regard to the production of new species by physico… chemical means may be repeated with still more justification in regard to the second problem of transformation； namely the making of living from inanimate matter。 The purely morphological imitations of bacteria or cells which physicists have now and then proclaimed as artificially produced living beings； or the plays on words by which； e。g。 the regeneration of broken crystals and the regeneration of lost limbs by a crustacean were declared identical； will not appeal to the biologist。 We know that growth and development in animals and plants are determined by definite although complicated series of catenary chemical reactions； which result in the synthesis of a DEFINITE compound or group of compounds； namely； NUCLEINS。
The nucleins have the peculiarity of acting as ferments or enzymes for their own synthesis。 Thus a given type of nucleus will continue to synthesise other nuclein of its own kind。 This determines the continuity of a species； since each species has； probably； its own specific nuclein or nuclear material。 But it also shows us that whoever claims to have succeeded in making living matter from inanimate will have to prove that he has succeeded in producing nuclein material which acts as a ferment for its own synthesis and thus reproduces itself。 Nobody has thus far succeeded in this； although nothing warrants us in taking it for granted that this task is beyond the power of science。
XV。 THE VALUE OF COLOUR IN THE STRUGGLE FOR LIFE。
By E。B。 POULTON。 Hope Professor of Zoology in the University of Oxford。
INTRODUCTION。
The following pages have been written almost entirely from the historical stand…point。 Their principal object has been to give some account of the impressions produced on the mind of Darwin and his great compeer Wallace by various difficult problems suggested by the colours of living nature。 In order to render the brief summary of Darwin's thoughts and opinions on the