CARADOCIAN CYSTIDEA FROM GIRVAK 501 



change, e.g. the partly covered or partly suppressed pectinirhombs of Pleurocystis. 

 In every detail of the anatomy we can detect a purposeful adaptation ; of none can 

 we assert that it is a blind exaggeration of tendencies previously set agoing ; concern- 

 ing all the observed changes, we can deny that they were the expression of a process 

 opposed to the environment. It were only the consideration of these creatures as 

 mere petrifactions, and not as organisms correlated with external nature, that could 

 use them in proof of a hopeless predestination. 



To lay due stress on the influence of the present is not, of course, to deny the 

 momentum obtained from the past, or the possible conflict of the future. To-day 

 cannot obliterate yesterday, nor does it always make the most serviceable preparation 

 for the morrow. Let us consider the Girvan fossils more closely in illustration of 

 these well-worn truths. 



§ 581. Turning first to the Heterostelea — to Dendrocystis, Ceratocystis, Cothurno- 

 cystis, Trochocystis, and their allies, and examining them in the light of the fresh 

 information herewith submitted, we recognise that they constitute a group in which 

 the assumption of an altered relation to the environment, in respect mainly to gravity, 

 gradually modifies those ancestral characters that were originally assumed in accord- 

 ance with the normal pelmatozoic mode of life. But this change never succeeds in 

 aff"ecting the fundamental constitution of the animal. Thus the trend and the amount 

 of modification can more readily be distinguished, and only need to be interpreted in 

 harmony with the habitual facts of Echinoderm physiology. We find, for instance, 

 the openings of the alimentary canal, whenever they can be identified, interrupting 

 the bilaterality, and thus aff"ording a clue to ancestral structure. On the other hand, 

 their positions are governed by a few obvious principles (§§ 231, 232), and these can 

 readily be applied even to so specialised a form as Cothurnocystis. In that genus, 

 for example, examination of all possibilities soon makes it clear that the situation of 

 the openings is consistent with only one relation in respect to gravity (§ 234) ; and 

 this being once admitted, all the other peculiarities of Cothurnocystis, and they are 

 many, receive a ready explanation. 



§ 582. It is not so easy to understand all the peculiarities of Dendrocystis. Let 

 us begin by examining the relation to gravity. " If we consider the growth of any 

 body with reference to its fixed hereditary form and also to its relations to gravity, 

 we find," writes A. Hyatt (1881), "that efi'ort must necessarily be in six directions." 

 These are grouped in three pairs : (1) vertical, a, up, against gravitation, and h, down, 

 with gravitation ; (2) lateral, a, to the right, h, to the left, for equilibrium ; (3) axial, 

 or in the direction of locomotion, a, forward, h, backward. If, now, we imagine some 

 form like Dendrocystis as erect on a vertical stem, we shall see that the lateral 

 extension bears no relation to gravity : it is meaningless. If, however, we imagine 

 such a form to have fallen over on its side, we readily interpret this bilateral spreading 

 in the horizontal plane as an eff"ort on the part of the animal to recover that symmetrical 

 relation with respect to gravity which has been lost by its new position. Such a mode 



