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Although Dr. Zuckerman's new book is of less immediate interest to psychologists than his Social Life of Monkeys and Apes, it nevertheless adds an important contribution to the data required by all who are interested in the phylogeny of human impulses. Thus, for example, it is of psychological significance to learn that our ancestors seem to have had no rutting season since the eocene period (30 to 50 million years ago) at least; for uninterrupted breeding is characteristic of both the new and old-world primates, which were already distinct at this time. Indeed, the fact that, of all the primates, a breeding season exists only among the Mascarine and probably the African lemurs suggests that 'this peculiarity of the Lemuroidea may be regarded as a specialization away from the basal breeding habit of the primate stock' (pp. 163–164).
The menstrual cycle, on the other hand, is peculiar to, and common to all families of, the Catarrhine division (old world apes and monkeys), so that it was probably developed since their divergence from the new world monkeys (eocene) and before their differentiation among themselves (late eocene and early oligocene). The sexual skin swelling, which makes the female especially attractive to the male at a certain point in her oestral cycle, is present in the Pongidæ (gorilla, chimpanzee, orang) and the old-world monkeys, but absent in gibbons and man. It would be interesting to learn whether anything that fulfils the same function exists in gibbons and has ever existed in our own family.
A further stimulating problem is suggested by the failure to detect any variation in intellectual capacity throughout the entire sub-order Pithocoidea (apes and monkeys). If man's intelligence is so unique, to what (? traumatic) factors is it due?
Lastly there is Dr. Zuckerman's interesting suggestion that 'overt polygyny declined from the moment when primitive man became predominantly a hunting and food-sharing animal. … polygynous animals could not ha