ioo INVERTEBRATE PALAEONTOLOGY 



its acme. On the other hand, if the mid-zonal plates are 

 less elaborate than some nearer the peristome, the species 

 is regressive, in that the culmination of morphogenetic 

 development is reached, and passed, before the animal is 

 fully grown. Great care is needed in interpretation of 

 records of this type, since secondary specialization 

 due to local requirements often obscures the original 

 structure. But with due precaution, study of mor- 

 phogenetic stages of development may often lead 

 to deductions that can be verified by independent 

 evidence. 



(Ill) SPECIALIZATION AND REGRESSION 



Arguing from the observed features of ontogeny, it is 

 possible to conceive that a group of animals, at its in- 

 ception, may be capable of improvement, the stage of 

 perfection being attained when general efficiency is 

 acquired. But it is impossible to maintain separation 

 between the organisms and their environment. Organs 

 that would do good service in rocky surroundings would 

 probably prove worthless on sand or ooze ; and, since 

 groups are liable to changes of habitat, either of their 

 own accord or owing to physiographical influence, the 

 efficiency of one period may become the imperfection of 

 another. The Echinoidea may be cited in illustration 

 of this phenomenon. A large proportion of the Palae- 

 ozoic " Sea-Urchins " had flexible tests, apparently 

 suited for life amid yielding materials ; some modern 

 members of the group, with similar habitat, show 

 corresponding characters. During Triassic times, the 

 Echinoids seem to have changed their mode of life, 

 choosing reefs and rocks as their environment. Exposure 

 to waves demanded strength and rigidity of fabric, and 

 " Perischoechinoid " flexibility was abandoned. Through* 



