4i8 



NA TURE 



[September 3, 1908 



answered, this is also true of a machine the action of 

 which depends on how it was made, and in a less degree 

 on the treatment it has received during use. But in living 

 things this last feature in behaviour is far more striking, 

 and in the higher organisms past experience is all-important 

 in deciding the response to stimulus. The organism is a 

 plastic machine profoundly affected in structure by its own 

 action, and the unknown process intervening between 

 stimulus and reaction (on which the indirectness of the 

 response depends) must have the fullest value allowed it as 

 a characteristic of living creatures. 



For the zoological side of biology a view similar to that 

 of Pfeffer has been clearly stated by Jennings ' in his 

 admirable studies on the behaviour of infusoria, rotifers, 

 &c. He advances strong arguments against the theories 

 of Loeb and others, according to which the stimulus acts 

 directly on the organs of movement ; a point of view 

 which was formerly held by botanists, but has since given 

 place to the conception of the stimulation acting on the 

 organism as a whole. Unfortunately for botanists these 

 movements are by the zoologists called tropisms, and are 

 thus liable to be confused with the geotropism, helio- 

 tropism, &c., of plants : to these movements, which are 

 not considered by botanists to be due to direct action of 

 stimuli, Locb's assumptions do not seem to be applicable. 



Jennings's f>osition is that we must take into considera- 

 tion what he calls " physiological state, i.e., ' the varying 

 internal physiological conditions of the organism, as dis- 

 tinguished from permanent anatomical conditions.'" 

 Though he does not claim novelty for his view, I am not 

 aware that it has ever been so well stated. External 

 stimuli are supposed to act by altering this physiological 

 state ; that is, the organism is temporarily transformed into 

 what, judged by its reactions, is practically a different 

 creature. 



This may be illustrated by the behaviour of Stentor, 

 one of the fixed infusoria." If a fine jet of water is 

 directed against the disc of the creature, it contracts " like 

 a flash " into its tube. In about half a minute it expands 

 again and the cilia resume their activity. Now we cause 

 the current to act again upon the disc. This time the 

 Stentor does not contract, which proves that the animal 

 has been in some way changed by the first stimulus. This 

 is a simple example of "physiological state." When the 

 Stentor was at rest, before it received the first current 

 of water, it was in state i, the stimulus changed state i 

 into state 2, to which contraction is the reaction. When 

 again stimulated it passed into state 3, which does not 

 produce contraction. 



We cannot prove that the contraction which occurred 

 when the Stentor was first stimulated was due to a change 

 of state. But it is a fair deduction from the result of the 

 whole experiment, for after the original reaction the 

 creature is undoubtedly in a changed state, since it no 

 longer reacts in the same way to a repetition of the original 

 stimulus. 



Jennings points out that, as in the case of plants, spon- 

 taneous acts are brought about when the physiological 

 state is changed by unknown causes, whereas in other cases 

 we can point to an external agency by which the same 

 result is effected. 



Morphological Changes. 



Let us pass on to the consideration of the permanent 

 or morphological changes and the stimuli by which they 

 are produced, a subject to which, in recent years, many 

 workers have devoted themselves. I need only mention 

 the names of Vbchting, Goebel, and Klebs among botanists, 

 and those of Loeb, Herbst, and Driesch among zoologists, 

 to remind you of the type of research to which I refer. 



These morphological alterations produced by changes in 

 environment have been brought under the rubric of reaction 

 to stimulation, and must be considered as essentially 

 similar to the class of temporary movements of which 1 

 have spoken. 



The very first stage in development may be determined 



^ H, S. Jennings, " Contributions to the .Study of the Behaviour of the 

 Lower Organisms." Carnegie Institution, 1904. p. iii. 



^ Jennings, " Behaviour of the Lower Organisms," 1906, p. 170. 



NO. 2027, VOL. 78] 



by a purely external stimulus. Thus the position of the 

 first cell-wall in the developing spore of Equisetum is deter- 

 mined by the direction of incident light.' In the same 

 way the direction of light settles the plane of symmetry 

 of Marchantia as it develops from the gemma." But the 

 more interesting cases are those where the presence or 

 absence of a stimulus makes an elaborate structural differ- 

 ence in the organism. Thus, as StahP has shown, beech 

 leaves developed in the deep shade of the middle of the 

 tree are so different in structure from leaves grown in full 

 sunlight that they would unhesitatingly be described as 

 belonging to different species. Another well-known case 

 is the development of the scale-leaves on the rhizome 

 of CircEea into the foliage leaves under the action oJ 

 light." 



The power which the experimenter has over the lower 

 plants is shown by Klebs, who kept Saprolcgnia mixta, a 

 fungus found on dead flies, in uninterrupted vegetative 

 growth for six years ; while by removing a fragment of 

 the plant and cultivating it in other conditions the . 

 reproductive organs could at any time be made to 

 appear.'' 



Chlamydomoiias media, a unicellular green alga, when 

 grown in a 04 per cent, nutrient solution continues to 

 increase by simple division, but conjugating gametes are 

 formed in a few days if the plant is placed in pure water 

 and kept in bright light." Numberless other cases could 

 be given of the regulation of form in the lower organisms. 

 Thus Sporodinia grown on peptone-gelatine produces 

 sporangiferous hypha, but on sugar zygotes are formed. 

 Again, Protosiphon botryoides, if grown on damp clay, can 

 most readily be made to produce spores by transference to 

 water either in light or in darkness. But for the same 

 plant cultivated in Knop's solution the end can best be 

 obtained by placing the culture in the dark.' Still these 

 instances of the regulation of reproduction are not so 

 interesting from our point of view as some of Klebs' later 

 results." Thus he has shown that the colour of the flower 

 of Campanula trachelium can be changed from blue to 

 white and back again to blue by varying the conditions 

 under which the plant is cultivated. Again, with Semper- 

 vivum ° he has been able to produce striking results — 

 e.g., the formation of apetalous flowers with one instead 

 of two rows of stamens. Diminution in the number of 

 stamens is a common occurrence in his experimental 

 plants, and absolute loss of these organs also occurs. 

 Many other abnormalities were induced, both in the 

 stamens and in other parts of the flowers. 



There is nothing new in the character of these facts ; "* 

 what has been brought to light (principally by the work 

 of Klebs) is the degree to which ontogeny is controllable. 

 We are so much in the habit of thinking of the stable 

 element in ontogeny that the work of Klebs strikes us 

 with something of a shock. Most people would allow that 

 change of form is ultimately referable to changed con- 

 ditions, but many of us were not prepared to learn the 

 great importance of external stimuli in ontogeny. 



Klebs begins by assuming that every species has a 

 definite specific structure, which he compares to chemical 

 character. Just as a substance such as sulphur may 

 assuine different forms under different treatment, so he 

 assumes that the specific structure of a plant has certain 

 potentialities which mav be brought to light by appropriate 

 stimuli. He divides the agencies affecting the structure 

 into external and internal conditions, the external being 

 supposed to act by causing alterations in the internal 

 conditions. 



It will be seen that the scheme is broadly the same as 

 that of Pfeffer for the case of the movement and other 

 temporary reactions. The internal conditions of Klebs 

 correspond also to the " physiological state " of Jennings. 



From what has gone before, it will be seen that the 



1 Ptahl, Ber.d. Bol. Grs . 1885, p. 334. 



2 Pfeffer, in Sachs' Arheitcn, i. p. 92. 



•' Ji-ftaische ZeUschr.. 1883, p. 162. ^ Goebel in Bot. Zeitung. 18S0. 



2 Winkiirliche F.ntwick., p. 27. 6 Klebs, Bedingiiugcti, 1896, p. 430. 



" Biel. Ceniralbl.. IQ04. rp. 451-3. 



'^ Jahrh. f. viss. Bat., xlii. tqo6, p. 162. 



■' Ahhandl. N aturforsch - Ces. zu Halle, xxv., 1906, pp. 31, 34, &c. 



10 See the great collection of facts illustratine the " direct and definite 

 .^ction of the external conditions of life" in "Variation of Animals and 

 Plants," ii. 271. 



