INVERTEBRATA, CRYPTOaAMIA, MICROSOOPYj ETC. 597 



degenerate vertebrates, as the barnacles are degenerate crustaceans. 

 The identity of the tadpole of the ascidian and the tadjjolc of the frog 

 is illustrated by figures representing the external ai)poaranco and the 

 chief internal organs, together with others, showing how the degenera- 

 tion proceeds which the ascidian tadpole has to go through to arrive 

 at the adult structure. 



The chief causes of structural degradation are (1) parasitism, (2) 

 fixity or immobility (as in the adult bai'nacle and ascidian), (3) vege- 

 tative nutrition (as in the green Planarian worms), and (4) excessive 

 reduction in size (exemplified in the Rotifers, Ostracoda, and Polyzoa). 

 Where the conditions are present degeneration may be suspected even 

 in the absence of any confirmatory embryological evidence. 



Degenerative evolution is not limited to zoology, but is applicable 

 to botany as w'cll, as it clearly offers an explanation of many vegetable 

 phenomena, and is already admitted as the explanation of facts con- 

 nected with the reproductive process in the higher plants. The yeast- 

 plant is in all probability a degenerate floating form derived from a 

 species of Mucor. 



Animal Development.* — Professor Schafer, in his lectures on 

 Animal Development delivered at the Royal Institution, thus formu- 

 lates some of the general results arrived at from a consideration of 

 the facts discussed : — 



(1) If we compare the processes of development of any two 

 animals, from sponges upwards, we find complete correspondence up 

 to a certain point ; from which point they may diverge from one 

 another. This point is sometimes placed near the bottom of the 

 development-scale, sometimes near the top ; or it may be in any inter- 

 mediate position. 



(2) Development is essentially localization of function and con- 

 comitant or consequent modification of structure ; such modification 

 being accompanied by segregation of the cells concerned with the 

 function localized. 



(3) The i)ath of development of all the more important of these 

 BCgi'cgated parts is the same up to a certain point in the development 

 of each segregation. From this point it may, in any animals or group 

 of animals, diverge from the rest, or may remain stationaxy, whilst in 

 the others specialization and modification progress further. 



(4) The various stages or phases of development of an animal, as 

 well as of its specialized parts, are often found to correspond with 

 cither permanent or transient conditions of animals lower in tho 

 scale. 



(5) Since the phases of development of individual animals are often 

 seen to be rejnesentations of the permanent conditions which are met 

 with in a serius of animals belonging to lower grades of organization, 

 it is impossible not to infer that these successive phases in the deve- 

 lopment of the individual represent simihir i)hases in the process of 

 formation or development of the race to which tho individual belongs. 



* 'Qiiiirt. Joiirn. Mirr. Si-i.,' xx. (ISiO) [>. 202. (Coiitiiiiiing Ihc bubstuiico ol" 

 the luiil two of the twelve Iccturei;.) 



