Instinet-reactions of animals 263 
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 the 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. 
