5 83 TEE POPULAR SCIENCE MONTE LY 



lies (Group I.) in forming a hydroxide which is soluble in water and 

 strongly alkaline in reaction. 



Other examples, concerning which it is unnecessary to enter into 

 detail, are the resemblances between phosphorus and sulphur; between 

 beryllium and aluminum ; between manganese and chromium ; between 

 boron, carbon and silicon; between gold and the platinum metals. It 

 will be observed from the zoological examples above cited that the mem- 

 bers of a phylum, while showing a greater or less similarity to each 

 other, will often markedly resemble members of different phyla. The 

 examples I have given show that a similar phenomenon is often char- 

 acteristic of the elements of a family — the elements compared are in 

 most cases similar to the other elements of the same family, while having 

 at the same time the points of resemblance with each other described; 

 and since the relationships referred to between distinct groups of organ- 

 isms are believed to indicate a common origin, we may, perhaps, consider 

 the analogous phenomena among the elements as of the same import. 



Did space permit, other analogies might be pointed out between the 

 Periodic and the zoological classifications ; but enough has already been 

 indicated to show that the Periodic classification possesses the main 

 characteristic features of the zoological classification. 15 Now, the fact 

 that these characteristics of the latter system are in themselves an indi- 

 cation of organic evolution suggests that the Periodic classification may 

 be regarded in the same light, as I have already indicated. This sug- 

 gestion is strengthened by the further evidence now to be considered. 



The Homologue of the Embryological Evidence ; the Phenomena 



of Kadioactivtty 



The study of comparative embryology has brought to light certain 

 facts which constitute important evidence of organic evolution; for 

 many of the higher animals, in their immature forms, pass through 

 stages in which they resemble more or less the adult forms of other 

 animals, lower in the scale of differentiation. Moreover, animals of 

 distinct but related species, in the progress of their development, often 

 show marked similarities of structure. Von Baer 



found that in its earliest stage, every organism has the greatest number of 

 characters in common with all other organisms in their earliest stages; that at 

 a stage somewhat later, its structure is like the structure displayed at corre- 

 sponding phases by a less extensive multitude of organisms; that at each sub- 

 sequent stage, traits are acquired which successively distinguish the developing 

 embryo from groups of embryos that it previously resembled — thus step by step 

 diminishing the class of embryos which it still resembles; and that thus the class 

 of similar forms is finally narrowed to the species of which it is a member. 18 



" The periodicity factor in the classification of the elements will be consid- 

 ered later (p. 97). 



18 Von Baer, quoted by Spencer, "Principles of Biology," Vol. I., p. 365. 



