CHARACTER EVOLUTION 241 



conditions inherent in that which varies. It is quite conceiv- 

 able that every species tends to produce varieties of a Hmited 

 number and kind, and that the effect of natural selection is to 

 favor the development of some of these, while it opposes the 

 development of others along their predetermined lines of 

 modification."^ It is true that the variations of the organ- 

 ism are in some respects limited in the heredity-chromatin, as 

 Huxley imagined; on the contrary, every part of a mammal 

 may exhibit such plasticity in course of geologic time as enables 

 it to pass from one habitat zone into another, and from that 

 into still others until finally traces of the adaptations to pre- 

 vious habitats and anatomical phases may be almost if not 

 entirely lost. The heredity-chromatin never determines be- 

 forehand into what new environment the lot of a mammal 

 family may be cast; this is determined by cosmic and plane- 

 tary changes as well as by the appetites and initiative of the 

 organism (p. 114). For example, one of the most remarkable 

 instances which have been discovered is that of the reversed 

 aquatic adaptation of Z en gl odour first terrestrial, then aquatic, 

 in succession a dog-like, a fish-like, and finally an eel-like 

 mammal. These peculiar whales (Archasoceti) appear to have 

 originated in the littoral and pelagic waters of Africa in Eocene 

 time from a purely terrestrial ancestral form of mammal 

 (allied to Hycsnodon), in which the body is proportioned like 

 that of the wolf or dog, and this terrestrial mammal in turn 

 was descended from a very remote arboreal ancestor. Thus 

 in its long history the Zeuglodon passed through at least three 

 habitat zones and as many life phases. 



Yet in another sense Huxley was right, for palaeontolo- 



1 Huxley, Thomas, 1893, p. 223 (first published in 1878). 



-Zeuglodon itself is a highly specialized side branch of the primitive toothed whales. 

 The true whales may have arisen from the genera Protocetus, probably ancestral to the 

 toothed whales, and Patriocctiis which combines characters of the zeuglodonts and 

 whalebone whales. 



