TEE PRIMARY FACTORS OF EVOLUTION. 323 



radiated body is also due to a partially fixed position, and 

 the radiate or star-shaped body of most Echinoderms is 

 due to their fixed condition or to their limited locomotion 

 in an indefinite direction, thongb their larvae are bilateral, 

 and the worm-like shape of Holothurians shows that they 

 originated from some kind of worm. In worms the fore 

 and aft and bilateral symmetry was inonced in some gas- 

 triila or embryo worm which crept straight ahead. 



We will now state some of the more obvious facts which 

 tend to show that it was the changes in the action of the 

 primary factors of evolution which gave the impetus to this 

 or that change in habit or function, which brought about 

 such changes in the shape and structure of organs, as to 

 give rise first to variations, then to species, and in the be- 

 ginning and throughout geological time to new genera, 

 families, orders, classes, and branches or phyla. 



Chemical Change in the Medium. — The brine-shrimp 

 (Artemia) is evidently derived from the fresh-water Bran- 

 chipus (Fig. Ill), becoming dwarfed by its life in brine. 

 There are no marine Phyllopods. A Russian observer has 

 shown that it was possible to artificially raise a brood of 

 the brine-shrimp {Artemia inilhansenii) from Artemia 

 salina which lived in suit water of 4" Beaume by gradually 

 raising the percentage of salt to 35° B. This transforma- 

 tion occurs very gradually after several generations. He 

 observed the same process in nature near Odessa: "A 

 dam which divided a lake containing salt water of 4° B. 

 from another where the water marked 25° B, gave way in 

 the year 1871, so that the density of the water in the lower 

 lake fell to 8° B. At the same time numberless individu- 

 als of Artemia salina were carried through to the lower 

 lake by the flood, and there they soon settled and propa- 

 gated. After the dam was repaired the saltness of the 

 water in the lower lake naturally increased again; in 1872 

 it had risen to 14° B., in 1873 to 18° B., and by the end of 

 September, 1874, it had reached its old mark of 25° B. 

 During this period the Artemia salina that had migrated 



