2 8 NATURAL SCIENCE [July 



gulf which separates the invertebrates from the lowest vertebrates, 

 the cyclostomes, can be bridged. I do not see how it is possible to 

 demonstrate either the strength or the weakness of the particular 

 bridge here sketched. It is enough if it is merely kept in mind as 

 a hypothesis, that is by those to whom it appeals. Beyond sug- 

 gesting this possible method of bringing about the more fundamental 

 morphological changes which transformed the invertebrate into the 

 vertebrate, the hypothesis does not profess to go ; the subsequent 

 development of skeleton, limbs, jaws, teeth, &c, belongs to vertebrate 

 morphology as such. 



One possible test, viz., that of direct transition forms, is entirely 

 lacking. If anything, however, this favours our argument. For 

 if there be any truth in our suggestion we could hardly expect to 

 find transition forms, at least certainly not fossilised, for such 

 hirudinean ancestors would naturally be soft-bodied, and, by the 

 time the notochord was developed in the manner sketched, they 

 would be no longer hirudineans but primitive cyclostomes. 



One class of vertebrate, however, we do find, which is apparently 

 lower than the cyclostomes — Ampkioxus and the Tunicata. Though 

 these have long been recognised as related to the vertebrates, their 

 position in the chain of evolution has been matter of endless con- 

 troversy, for the obvious reason that the order of the chain is not 

 known. It seems to me that the line of development from the in- 

 vertebrate hirudinean to the vertebrate cyclostoine which is here 

 suggested would assign them a natural position. They are not true 

 links in the ascending chain, but forms which have branched off and 

 become differentiated, again, be it specially noted, in adaptation 

 to a new method of feeding. We postulated for our earliest 

 ancestors when they were just turning into vertebrates, an active 

 free-swimming life, catching and devouring food with gaping jaws, 

 and consequently with a stream of water Mowing through the 

 pharynx and out at the gill-slits, kept up by rapid swimming 

 through the water. This constant stream of water through the 

 pharynx would carry along with it innumerable small particles of 

 food, which might with advantage be caught and turned down the 

 oesophagus. It seems probable that all the early vertebrates de- 

 veloped an organ for this purpose, the endostyle, the particles 

 perhaps attracted by and adhering to a slimy secretion which was 

 then worked by ciliary motion into the oesophagus. Now, either in 

 special regions where these small particles were unusually plentiful, 

 while larger prey requiring great exertions and the use of teeth was 

 scanty, or among very young forms not yet strong enough to attack 

 prey with mouth and teeth, 1 this passive method of feeding was 

 nearly sure to be adopted by a certain number of individuals as 



1 The larval amphibian has acquired a browsing herbivorous diet. 



