We see, also, that any important change in treatment and surroundings, if not sufficient to kill, would and does tend to produce not only variability but sterility, as part of the same story and for the same reason—namely, default of memory; this default will be of every degree of intensity, from total failure, to a slight disturbance of memory as affecting some one particular organ only; that is to say, from total sterility, to a slight variation in an unimportant part. So that even _the slightest conceivable variations should be referred to changed conditions_, _external or internal_, _and to their disturbing effects upon the memory_; and sterility, without any apparent disease of the reproductive system, may be referred not so much to special delicacy or susceptibility of the organs of reproduction as to inability on the part of the creature to know where it is, and to recognise itself as the same creature which it has been accustomed to reproduce. Mr. Darwin thinks that the comparison of habit with instinct gives “an accurate notion of the frame of mind under which an instinctive action is performed, but not,” he thinks, “of its origin.” “How unconsciously,” Mr. Darwin continues, “many habitual actions are performed, indeed not rarely in direct opposition to our conscious will! Yet they may be modified by the will or by reason. Habits easily become associated with other habits, with certain periods of time and states of body. When once acquired, they often remain constant throughout life. Several other points of resemblance between instincts and habits could be pointed out. As in repeating a well-known song, so in instincts, one action follows another by a sort of rhythm. If a person be interrupted in a song or in repeating anything by rote, he is generally forced to go back to recover the habitual train of thought; so P. Huber found it was with a caterpillar, which makes a very complicated hammock. For if he took a caterpillar which had completed its hammock up to, say, the sixth stage of construction, and put it into a hammock completed up only to the third stage, the caterpillar simply re-performed the fourth, fifth, and sixth stages of construction. If, however, a caterpillar were taken out of a hammock made up, for instance, to the third stage, and were put into one finished up to the sixth stage, so that much of its work was already done for it, far from deriving any benefit from this, it was much embarrassed, and in order to complete its hammock, seemed forced to start from the third stage, where it had left off, and thus tried to complete the already finished work.”