4 
ZOOLOGICAL LITERATUllE. 
He devotes two chapters to preliminary inquiries concerning 
the Origin of species/^ Specific centres/^ and Modes 
of dispersal of species/^ He accepts tlie theory that species 
are not produced by independent creation, hut that, under 
the operation of a general law, the germs of organisms pro- 
duce new forms difterent from themselves when particular cir- 
cumstances call the law into action. This theory appears to 
him to furnish a satisfactory explanation of the homologies in 
structure and of the relationships between species. He thinks 
that species are not continuously, unintermittingly changing, 
that there is a law of inertia securing their stability, but that a 
change in the conditions under which species live is always 
followed by a modification of specific characters. These ideas 
of the author may be illustrated by the way in, which he ac- 
counts for the existence of three marine Mammalia in rivers, 
viz. Platanista g angelica in the Ganges, PI. inili in the Indus, 
and amazonica in the Amazons (pp. 213-215). He says 
that the two Indian species are so closely allied that they must 
have taken their origin from a common and not very distant 
ancestor; and as an animal never seeks a change producing the 
development of a new species {inertia being strong to keep it 
where it is), we must argue that here the change from salt to 
fresh water must have been forced upon the Dolphins : — 
1, In geological time the desert country through which the 
Indus flows was covered by a sea inhabited by truly marine 
genera of Dolphins. 
2. As the land rose, a salt-lake remained, shut off from the 
sea by the elevation of the coast between Bombay and Kur- 
rachee. 
3 & 4. Into this lake flowed the waters supplying the Indus 
and other rivers, as well as the sources of the Ganges, changing 
the salt into fresli water. 
5. This change was not so rapid as to destroy the Dolphins, 
but sufficiently so to induce a change in the species. 
6. The dividing watershed which now separates the sources 
of the Ganges from the sources of the Indus had not yet been 
sufficiently elevated to divide the two, and as soon as the lake 
was full to overflowing it overflowed, and the waters escaped 
into the line of the Ganges. There would then only be one 
great river in the nortli of India, and that the Ganges. By the 
time this happened, the transforipation of the Dolphin into the 
Platanista had been completed : it may have been either Plata- 
nista indi or P. g angelica that was produced, or it may have 
been a common ancestor of both. When this happened, the 
Platanista, whatever its species, would inhabit both the lake and 
the Ganges; but tlicy could not go back to the sea, via the 
Ganges, for by this time they liad been changed into fresh- 
water species. 
