Instinct-reactions of animals 2(53 



lateral branches on the lower side give rise to an altogether different 

 kind of organ, namely, to roots, and these roots grow indefinitely in 

 length and attach themselves to solid bodies ; while if the stem had 

 remained in its normal position no further growth would have 

 occurred in the lateral branches. From the upper side of the hori- 

 zontal stem new stems grow out, mostly directly from the original 

 stem, occasionally also from the short lateral branches. It is thus 

 possible to force upon this hydroid an arrangement of organs which 

 is altogether different from the hereditary arrangement. The writer 

 had called the change in the hereditary arrangement of organs or the 

 transformation of organs by external forces heteromorphosis. We 

 cannot now go any further into this subject, which should, however, 

 prove of interest in relation to the problem of heredity. 



If it is correct to apply inferences drawn from the observation on 

 the frog's egg to the behaviour of Antennularia, one might conclude 

 that the cells of Antennularia also contain non-miscible substances of 

 different specific gravity, and that wherever the specifically lighter 

 substance comes in contact with the sea-water (or gets near the 

 surface of the cell) the growth of a stem is favoured ; while contact 

 with the sea- water of the specifically heavier of the substances, will 

 favour the formation of roots. 



VI. The Experimental Control of Animal Instincts. 



(a) Experiments on tlie mechanism of heliotropic reactions in 

 animals. 



Since the instinctive reactions of animals are as hereditary as 

 their morphological character, a discussion of experiments on the 

 physico-chemical character of the instinctive reactions of animals 

 should not be entirely omitted from this sketch. It is obvious that 

 such experiments must begin with the simplest type of instincts, if 

 they are expected to lead to any results ; and it is also obvious that 

 only such animals must be selected for this purpose, the reactions of 

 which are not complicated by associative memory or, as it may 

 preferably be termed, associative hysteresis. 



The simplest type of instincts is represented by the purposeful 

 motions of animals to or from a source of energy, e.g. light ; and it is 

 with some of these that we intend to deal here. When we expose 

 winged aphides (after they have flown away from the plant), or 

 young caterpillars of Porthesia chrysorrhoea (when they are aroused 

 from their winter sleep) or marine or freshwater copepods and many 

 other animals, to diffused daylight falling in from a window, we notice 

 a tendency among these animals to move towards the source of light. 



