C— GEOLOGY 73 



a wholly gratuitous embellishment. It may well have had a significance 

 that will be mentioned in the following paragraph ; but taken on its own 

 merits it seems to show a trend of evolution as automatic as unnecessary. 



The thickness of the test of Micraster is another progressive character. 

 M. corbovis has a very thin test in proportion to its not inconsiderable 

 size, while at the upper extreme M. rostratus has a smaller but much thicker 

 test. Again the chronological sequence is almost perfect from thin to 

 thick. In this case various reasons can be postulated for a change that 

 cannot have been wholly without influence on the bionomics of the animal. 

 It has been suggested that life in an environment of calcareous ooze made 

 absorption and secretion of calcite a sort of disease. This suggestion, 

 however, leaves quite unexplained the delicacy of the tests of several of 

 Micraster s contemporaries and associates, and the extreme flimsiness of 

 many recent sea-urchins that live in comparable surroundings. Explana- 

 tions based on variation of depth of the Chalk sea are no more satisfying, 

 for that quality undoubtedly fluctuated, while the thickening of the test 

 is steadily progressive. There is, in short, no sign that this character was 

 either enforced by the environment or advantageous to the animal, but 

 it developed notwithstanding. By analogy with the case of the petaloid 

 ornament, we might suggest that the building of a test (that is, a tend- 

 ency to secrete calcite) was a quality which, once started, continued 

 regardless of convenience or necessity. Perhaps this is no analogy, but 

 all part of the same story ; for the packing of the interporiferous tract 

 with granules until it is flush with the surface of the test may well indicate 

 a storage of surplus calcite in a place where it would be least in the way. 

 Since a similar embarrassment of mineral wealth seems to overwhelm a 

 large proportion of the organisms that come within palaeontological reach, 

 it may be taken as provisionally true that mineralisation, however useful 

 it may be in moderation, becomes gradually a disease. 



The third character in the test of Micraster that we can select for 

 analysis is that of the labrum, a shovel-like prolongation of the ' lower ' 

 lip of the peristome. In this feature progressive change is again in evi- 

 dence. Low-zonal species have a scarcely recognisable labrum, while in 

 high-zonal forms the structure may be so strongly developed as to project 

 beyond the anterior end of the body. There can be no question that, 

 for an animal with the habits of a Micraster, a well-developed scoop 

 is an aid to efficiency in feeding. The food-bearing ooze or silt is, in part 

 at least, directed towards the mouth by the anterior sulcus of the test, 

 and any device at the peristome to ensure its entry into the mouth would 

 avoid waste. It seems reasonable to conclude that some stimulus, 

 possibly that of friction during use, encouraged the initiation of a labrum, 

 and that the labrum continued, perhaps under the same stimulus, to in- 

 crease in size as one generation followed another. Such a suggestion 

 raises the spectre of the inheritance of acquired characters ; but this is 

 not the place to worry about warring hypotheses. The fact is that the 

 genus Micraster began with practically no labrum, and that that structure 

 progressively increased in size as time went on. But (and here is the 

 significant feature), after the labrum had reached adequate proportions, 

 it continued to grow until, in the latest types, it had so far outgrown 



D2 



