ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 76U 



Physiological Selection.* — Mr. G. J. Romanes finds certain diffi- 

 culties in regarding natural selection as a theory for the origin of 

 species, as it is rather a theory of the origin of adaptive structures. 

 He proposes to replace it by what he calls physiological selection, or 

 segregation of the fit. His view is based on the extreme sensitive- 

 ness of the reproductive system to small changes in the conditions of 

 life, and he thinks that variations in the direction of greater or less 

 sterility must frequently occur in wild species. If the variation be 

 such that the reproductive system, while showing some degree of 

 sterility with the parent form, continues to be fertile within the 

 limits of the varietal form, the variation would neither be swamped 

 by intercrossing, nor die out on account of sterility. When a varia- 

 tion of this kind occurs the physiological barrier must divide the 

 species into two parts. The author, in fine, regards mutual sterility 

 not as one of the effects of specific differentiation, but as the cause 

 of it. 



" F. J. B " in the ' Athenaeum ' f points out that naturalists have 

 long recognized that there are " morphological " and " physiologi- 

 cal " species. The former have their origin in men's minds, the 

 latter in a series of changes sufficient to affect the internal as well as 

 the external organs of a group of allied individuals. The " physio- 

 logical selection " of morphological species is a confusion, of ideas ; 

 that of physiological species a redundancy of terms. 



y. General. { 



Ancestry of the Chordata.§ —Mr. W. Bateson, in an essay on 

 this subject, commences by discussing the segmentation of Amphioxus 

 and the vertebrates as compared with that of annelids ; this is a 

 phenomenon that has been insisted on as proving the genetic affinity 

 of the two groups ; when it is investigated, however, we find that 

 greater or less repetition of various structures is one of the chief 

 factors in the composition of animal forms, that one or many organs 

 may be affected, and that this repetition may be at first irregular, 

 culminate in regularity, and again vary. The fact that the gonads are 

 so often repeated, tends to show that the repetition first arose in adult 

 life ; even at this present epoch, many repetitions still appear at a 

 late period only. 



In the Chordata it is to be noted that the mesoblastic plates are at 

 first unbroken, the medullary plate has no transverse divisions, and 

 the excretory ducts are single tubes with single openings. Further, it 

 is important to observe that of the characteristic organs of the 

 Chordata, the notochord is always unsegraented, while it is almost the 

 earliest organ formed ; the medullary plate does not exhibit any 

 serial repetition till the peripheral nerves arise, and in Amphioxus 



* Journ. Linn. Soc. Lond., xix. (1886) pp. 337-411. 

 t No. 3069 (Aug. 21, 1886) pp. 242-3. 



% This section is limited to papers which, while relating to Vertebrata, 

 have a direct or indirect bearing on Invertebrate also. 

 § Quart. Journ. Micr. 8ci„ xxvi. (1886) pp. 535-72. 

 Ser. 2. — Vol. VI. 3 E 



