Royal Institution. 507 



In man himself, the individual, zoologically speaking, is not a 

 state of man at any particular moment as infant, child, youth or 

 man ; but the sum of all these, with the imphed fact of their definite 

 succession. 



In this case, and in most of the higher animals, the forms or 

 states of the individual are not naturally separated from one another ; 

 they pass into one another, undistinguishably. 



Among other animals, however, nature draws lines of demarcation 

 between the different forms ; thus, among insects the individual takes 

 three forms, the caterpillar, the chrysalis, and the butterfly. These 

 do not pass into one another insensil)ly, but are separated by appa- 

 rently sudden changes ; each change being accompanied by a sepa- 

 ration of the individual into two parts. One part is left behind and 

 dies, it receives the name of a skin or cast ; the other part continues 

 the existence of the individual under a new form. 



The whole process is called Ecdysis : it is a case of what might be 

 termed concentric fission. 



The peculiarity of this mode of fission is — that of the two portions 

 into which the individual becomes divided at each moult, one is un- 

 able to maintain an independent existence and therefore ceases to be 

 of any importance ; while the other continues to carry on all the 

 functions of animal life and to represent in itself the whole indivi- 

 duality of the animal. From this circumstance there is no objection 

 to any independent form being taken for, and spoken of, as the whole 

 individual, among the higher animals. 



But among the lower animals the mode of representation of the 

 individual is different, and any independent form ceases, in many cases, 

 to represent the whole individual ; these two modes, however, pass 

 into one another insensibly. 



The best illustration of this fact may be taken from the develop- 

 ment of the Echinoderms, as it has been made known by the brilUant 

 discoveries of Prof. Miiller. 



The Echinus lividus stands in the same relation to its PluteuSy as 

 a butterfly to its caterpillar ; in the course of development only a 

 slight ecdysis takes place, the skin of the Pluteus becoming for the 

 most part converted into the skin of the Echinus, 



But in Asterias, the Bipinnaria which corresponds with the Plu- 

 teus, gives up only a portion of its integument to the developed 

 Asterias ; the remaining and far larger portion lives for a time after 

 its separation as an independent form. 



The Bipinnaria and the Starfish are as much forms of the same 

 individual as are the Pluteus and Echinus, or the caterpillar and but- 

 terfly ; but here the development of one form is not necessarily fol- 

 lowed by the destruction of the other, and the individual is, for a time 

 at any rate, represented by two co-existing forms. 



This temporary co-existence of two forms of the individual might 

 become permanent, if the Asterias, instead of carrying off the intes- 

 tinal canal of the Bipinnaria, developed one of its own ; and this is 

 exactly what takes place in the Gyrodactylus, whose singular deve- 

 lopment has been described by Von Siebold. 



33* 



