LINNEAN SOCIETY OF LONJJOX. 43 



Leptocardia and the Marsipobrancbia are M'itb no certainty repre- 

 sented in the fossil state. The}^ are derived from an ancestor far 

 more ancient than the Theromorplis, and any comparison of 

 existing forms, supposed to have been derived from this ancestor, 

 might well show vastly greater differences than between say 

 Primates and Lacertilia, or even Primates and Pisces. 



The weakest part of the MacBride-Goodrich argument the other 

 night lies in their consideration of Amphiodus as a simple primi- 

 tive A'^ertebrate. Whatever Amphioxxis may be, it is surely not in 

 the main stem of the Vertebral e descent, and it is certainly a verv 

 specialized form. To argue, as Goodrich did, that the presence of 

 priuiitive excretory cells (soleuocytes) in Amphioxus proves it to 

 be primitive, and related to the Annelids, comes to the same 

 thing as claiming that Phoronis is also an Annelid, because its 

 larva has similar cells. 



Examining both the above groups, and applying " every canon of 

 Biology," we must, I conceive, regard Ampjliioxus as equally typical 

 of regression as is any beast that exists in the Animal Kingdom, 

 while the Marsipobrancbia as typically show progression. 

 Looking at the groups from this point of view the Leptocardia 

 may be cast aside from our discussion as unprofitable, and we can 

 turn with certainty to considering tlie morphology of Marsipo- 

 brancbs for some guide to the evolution of Vertebrates. 



It is not my desire to draw your attention to the series of facts, 

 both physiological and morphological, discovered by Dr. Gaskell 

 in his extensive comparison of the higher Invertebrates with the 

 lower Vertebrates. They present an extraordinary series of 

 analogies and probabilities which cannot be lightly passed 

 over, and, even if his views be ultimately rejected by palaeonto- 

 logical discoveries, will for ever make Zoologists indebted to 

 him for drawing their attention to a fresh and broader aspect 

 in which to consider their science. Of his comparisons I would 

 particularly draw attention to that between the internal cartila- 

 ginous skeleton of Limulus and that of Animocoetcs, the skeleton 

 being a part which, judging from fossil and living Vertebrates, 

 seems to retain for the longest period traces of all its developments, 

 "earmarks," as Osborne terms them. I might refer also to the 

 infundibulum, the commissures of the brain, the thyroid, the 

 auditory apparatus, and the existence of giant fibres and cells in 

 the nervous system. By far the simplest way to explain this 

 extraordinary series of coincidences between the organs of different 

 forms is to suppose that they are due to a common inheritance, 



I would turn now rather to the difHculties which beset the view, 

 and by far the chief of these must be deemed to be that relating 

 to the alimentary canal. To get that of Petromyzon from that of 

 Ammocoetes we have an entirely new formation of quite startling 

 character. This is a fact, and accepting it as such we can proceed 

 with our minds moi'e open, I think, to consider how a gut in 

 Vertebrates came into existence. Professor MacBride is quite 



