C— GEOLOGY 71 



complex organisms the chances against a perfect repetition of the whole 

 combination of adult features are greater, and consequently recapitulation 

 is more likely to be less complete. Nevertheless, even in highly organised 

 animals limited recapitulation is very common. Odd examples of this 

 have alv^rays attracted attention, and indeed have been the basis upon 

 which earlier workers, not excluding Haeckel himself, founded their 

 belief in the general principle. But what was evidence for them can no 

 longer satisfy us. 



Localised Recapitulation. 



Another set of facts which bears upon the problem may now be briefly 

 discussed. They were first noticed by Jackson (1892), who grouped 

 them under the heading ' Localised Stages in Development,' and are 

 commonly shown in those parts of the body which are metamerically 

 repeated or are reproduced by budding. Since it is in such parts that 

 evidence is forthcoming only limited recapitulation may be expected. 



Good illustrative examples of localised recapitulation are found among 

 the echinoids, but unfortunately detailed stratigraphical evidence is 

 usually lacking. In the palaeozoic Lepidocentridae, which appear to be 

 the forerunners of the mesozoic echinoids other than the cidarids, the 

 ambulacrum is narrow and made up of low laterally elongated plates 

 pierced by a pair of pores. In the Jurassic genus Hemicidaris , as in all 

 the echinoids, new plates are added to the ambulacrum at its upper end. 

 These move downwards towards the equatorial belt of the test and 

 gradually assume the fully grown condition. The newly formed plates 

 in the upper part are like those fully grown plates of the palaeozoic genera 

 in form and number of pores. As they pass downwards the plates become 

 associated with one another first in pairs and then in threes having a 

 common outline and decorated by a large tubercle. In its early stages of 

 growth, therefore, each plate recapitulates the condition shown by the 

 same structure in the ancestral palaeozoic stock. Again, MacBride and 

 Spencer (1938) have recently drawn attention to the interesting fact that, 

 in the development of the ambulacral plates in certain modern forms, the 

 podial pores first appear as notches in the lower border of the plate, a 

 condition which characterises the adult condition in some ordovician 

 echinoids. 



The case of Micraster, though not so striking, is perhaps more valuable 

 because the work of Dr. Rowe has produced a well-authenticated evolu- 

 tional series, based upon very careful and detailed stratigraphical work. 

 Unfortunately, owing to the mode of growth of the echinoids, the develop- 

 mental evidence for the test as a whole is not forthcoming. So far as 

 I am aware, attention has not hitherto been given to the existence of 

 localised stages in the development of the ambulacra in these micrasters, 

 but an examination of the uppermost portion of any well-preserved speci- 

 men of a high zonal variety of M. prcecursor will show that it reproduces 

 very closely the condition shown by the fully developed portion of the 

 ambulacrum seen in low zonal varieties. 



