188 
FRANCIS H. HERRICK 
The parental or cyclical impulses of the American species are 
irregular at one important point only — the production of eggs — 
and any disastrous results likely to arise from this cause have been 
annulled by the development of new instincts in their young, 
exhibited in the climbing stage, and the preparation for it to be 
seen in the strong grasping reflex, precocious muscular develop- 
ment, the combing" instinct, and comparatively sudden ap- 
pearance of the feathers on the seventh or eighth day. 
5. CYCLICAL INSTINCTS 
The behavior of birds is determined and expressed by a large 
number of recurrent instincts, some of which are of commanding 
importance and most of which are of ancient origin. For the sake 
of simplicity, their instincts may be divided into two classes; 
namely: (1) The Continuous Instincts, which are necessary 
for the preservation of the individual, such as preying, fear, con- 
cealment, and flight; and (2) Cyclical Instincts, which are 
requisite for the maintenance of the race. By cyclical instincts 
we mean those discontinuous, recurrent tendencies to action 
which attend the reproductive cycle, and with this understand- 
ing it will be convenient to speak of parental instincts, without 
specification in each case.-^ 
The cyclical or parental instincts as a rule recur with almost 
clock-like precision, in spring or summer, in the northern hemi- 
sphere, and with repetitions within the breeding season in cer- 
tain species. Fear, which is primarily due to inheritance, is sub- 
ject to continual modification through experience, and may even 
lapse completely; it not only modifies the cyclical instincts, es- 
pecially at the time of their emergence, but ma}" be completely 
blocked and annulled by them at a later period. 
2^ For abstracts of earlier papers on this subject, see: Analysis of cyclical 
instincts of birds. Science, vol. 25, p. 725, 1907, and The blending and overlap 
of instincts, Science, vol., 25, p. 781, 1907; also Instinct and intelligence in birds. 
Pop. Science Monthly, vol. 76, pp. 532-536, and vol. 77, pp. 82-97, 122-141. 
New York, 1910. Growth curves and other records of cuckoos are given here. 
