384 METAMORPHOSIS 



Although, even now, physiological data on this subject are extremely 

 limited, still it is of the greatest importance that a brief summary of these 

 should be presented in treating of the physiology of form changes. Owing to 

 the great part which CHARLES DARWIN played in the founding of the Theory of 

 Descent it will be most convenient to start with a very brief consideration of 

 his conception of the origin of species, more especially as his views are now very 

 generally accepted. 



DARWIN (1860) compared the origin of species in nature to the origin of 

 races in cultivation. The formation of different breeds starts with individual 

 variation, that is to say, with the fact that the offspring of the same two parents 

 are not similar in all respects ; of this variation there can be no doubt. The 

 breeder selects for propagation only such organisms as exhibit a certain 

 desired peculiarity, and he expects this character to be transmitted to their 

 offspring in turn. The appearance of variations and their inheritance is entirely 

 a natural phenomenon ; what the breeder does is merely to select (artificial selec- 

 tion) for propagation certain definite individuals. DARWIN believed that a 

 process, similar in all respects to that carried out by the breeder, could be recog- 

 nized in nature. From each animal or plant so many offspring originate that 

 only a fraction of them are able to find the necessaries of life ; the rest ' succumb 

 in the struggle for existence'. If it be asked how this struggle for existence 

 brings about natural selection we believe DARWIN to be correct in his assumption 

 that all the better equipped individuals will have a better chance of remaining 

 in existence and producing progeny than those less well adapted. Whether 

 the organism is well or less well adapted depends on its capacity for making 

 use of all that is favourable in its surroundings and for guarding itself against 

 what is injurious. Let us imagine two seeds of the same species germinating in 

 close proximity to each other ; the one seedling produces its root system rather 

 more rapidly than the other and seizes on the water and nutritive salts in the 

 soil before the other succeeds in doing so. The result will obviously be that the 

 former plant will be successful in the struggle for existence, while the other will 

 be dwarfed. The same result will come about if differences arise in the rate of 

 growth of aerial organs, and a rapidly-growing seedling will deprive the other of 

 the essential light rays. If a seedling contains some poisonous materials which 

 may act as protective agents, and if it possesses some mechanical protective struc- 

 tures, such as raphides or thorns, it will be better protected from the attacks of 

 animals, and have a greater chance of handing on its characters to offspring 

 than the plant that is destitute of such. In nature, as in artificial cultivation, 

 only some individuals reach the propagative stage, and if they transmit their 

 characters then these will be gradually emphasized, and species must alter as they 

 become more and more adapted to their conditions of life. 



Considering now the Darwinian theory more in detail, it is obvious that we 

 must give careful attention to the three factors which are specially concerned, 

 viz. variation, heredity, selection. Let us begin with the last, and ask ourselves 

 how it affects species formation. In attempting to answer this question we 

 cannot avoid discussing briefly the significance of the term ' species '. The 

 idea of species is a purely abstract one ; in nature there are no species only 

 individuals. By a species we mean the totality of individuals which belong to 

 the same line and which preserve their same characters for successive genera- 

 tions. In nature, however, we know nothing of the genealogy of each individual, 

 and regard as a species all those plants which agree in all essential features and 

 live under as nearly as possible similar external conditions. Since it is obvious 

 that different botanists may hold entirely different views as to what constitutes an 

 essential and what a non-essential character, there arise allsorts of discrepancies in 

 the identification and naming of forms . These discrepancies are aggravated accord- 

 ing to the varying accuracy of the investigations carried out and the tendency of 

 the investigator to lay emphasis on general characters possessed in common by 



