78 SECTIONAL ADDRESSES. 



when subjected to the requisite conditions, as the flightless birds of 

 diverse origin found on ocean islands, the flightless and running rails, 

 geese, and other races of New Zealand, the Pleistocene Genyornis of 

 the dried Lake Gallabonna, which, as desert conditions came on, began 

 to show a reduction of the inner toe. Among mammals the legs and 

 feet have been modified in the same way in at least three distinct 

 orders or suborders, during different periods, and in widely separated 

 regions. Living marsupials in Australia have the feet modified accord- 

 ing to their mode of life, whether chmbiug on trees or running over 

 hard ground; and among the latter we find a series indicating how 

 the outer toes were gradually lost and the fourth digit enlarged. I 

 need scarcely remind you of the modifications that resulted in the 

 horse's hoof with its enlarged third digit, traceable during the Tertiary 

 Epoch throughout the Northern Hemisphere, whether in one or more 

 than one stock. I would, however, recall the fact that occasional 

 races, resuming from time to time a forest habitat, ceased to progress 

 along the main line. Lastly, there are those early hoofed animals 

 from South America, made known by Ameghino under the name 

 Litopterna, which underwent a parallel series of changes and attained 

 in Thoatheriuvi from the Upper Miocene of Patagonia a one-toed foot 

 with elongate metacarpals essentially similar to that of the horse. In 

 all these cases the correlation of foot-structure with mode of life (as 

 also indicated by the teeth) is such that adaptation by selection has 

 always been regarded as the sole effective cause. 



My colleague, Dr. W. D. Lang, has recently published a most 

 thoughtful paper on this subject (1919, Proc. Geol. Assoc, xxx. 102). 

 His profound studies on certain lineages of Cretaceous Polyzoa 

 (Cheilostomata) have led him to believe that the habit of secreting 

 calcium carbonate, when once adopted, persists in an increasing degree. 

 Thus in lineage after lineage the habit ' has led to a brilliant but 

 comparatively brief cai'eer of skeleton-building, and has doomed the 

 organism finally to e\x)lve but the architecture of its tomb.' These 

 creatures, like all others which secrete calcium carbonate, are simply 

 suffering from a gouty diathesis, to which each race will eventually 

 succumb. Meanwhile the organism does its best to dispose of the 

 secretion ; if usefully, so much the better ; but at any rate where it 

 will be least in tlie way. Some primitive polyzoa, we are told, often 

 sealed themselves up ; others escaped this self-immurement by turning 

 the excess into spines, which in yet other forms fused into a front 

 wall. But the most successful architects were overwhelmed at last 

 by superabundance of building-material. 



While sympathetic to Dr. Lang's diagnosis of the disea-se (for in 

 1888 I hazarded the view that in Cephalopoda lime-deposition was 

 uncontrollable by the animal, and that its extent was inversely relative 

 to the rate of formation of chitin or other calcifiable tissue), still I 

 think he goes too far in postulating an ' insistent tendency. ' He 

 speaks of living matter as if it were the over-pumped inner-tube of 

 a bicycle tyre, ' tense with potentiality, curbed by inhibitions ' [of 

 the cover] and ' periodically breaking out as inhibitions are removed ' 

 ^y broken glass] . A race acquires the lime habit or the driixk habit, 



