3 20 SCIENCE PROGRESS. 



ordinary forms of metazoa is impossible for mechanical 

 reasons, and therefore special individuals of a form simple 

 enough to admit of conjugation are produced. These 

 special individuals are the ovum and spermatozoon. 



The explanation is extremely ingenious and there is 

 nothing unreasonable in it, but one cannot say that it is 

 altogether acceptable at first sight. It would have been 

 more satisfying if Mr. Sedgwick had marshalled some of 

 the facts relative to the sexual reproduction of some of the 

 lowest multicellular organisms and had shown their rela- 

 tion to his suggestion. A difficulty which at once occurs 

 to me is that in many plants asexual reproduction is 

 effected through the agency of a single cell. In fact, 

 before one can accept any solution of the question one 

 requires a very extensive and careful survey of all the facts 

 known about the reproduction of the lower plants. They 

 afford examples of every conceivable grade of the reproduc- 

 tive processes, and, once one begins to look into the subject, 

 hints as to the parting of the ways of sexual and asexual 

 reproduction occur to one at every step. The pity is that 

 the mere zoologist, who does not find such a fruitful field in 

 his own territory, is obliged to disinter the facts from the 

 load which the peculiarities of botanical terminology have 

 heaped upon them. 



It is quite possible, however, that such a survey would 

 afford strong support to Mr. Sedgwick's opinions, and if it 

 should do so they would in no way be inconsistent with 

 the ideas which I have put forward, but would rather sup- 

 port them. 



A word in conclusion for those who will reproach me 

 for having attempted to frame a chemico-physical theory 

 of organic growth, and for having used such phrases as 

 " complex chemical constitution," " exchange of chemical 

 material," etc., without assigning any distinct meaning 

 to them. I admit that our knowledge on the subject 

 is rather inadequate, and that I have used obscure phrases 

 to express relations which are in themselves obscure. If 

 one attempts to lift the veil of obscurity one must inevitably 



