282 
HAROLD’S DISCUSSIONS. 
tion, and of reproduction, bj light or heat, or some 
other external force. 
Apparently at the same time with the rhizopods 
there came into being the types of invertebrate life, 
and they seem to have been perfectly adapted to 
their environment. Had they ancestors which con¬ 
stituted the connecting link between themselves and 
the unicellular rhizopods ? If so, where were they ? 
Did their bodies leave no molds in the mud to be 
infiltrated, no substance to become petrified ? If so, 
why have they not been found ? Did they chance 
to live only in those parts of strata that have not 
yet been unearthed ? It may be, but it seems un¬ 
likely. 
The abundant forms of life continued for millions 
of years, with very slight variations here and there. 
Suddenly there appears a fish, a new type and perfect 
in all its parts; a higher type, having a spinal cord 
extending from the brain in a bony channel along the 
back. Where are the intergrading forms ? Again 
we inquire in vain. The few examples of a notochord 
afford us little satisfaction in our search. 
Trees, as well adapted to their age as are the palms 
to the tropics, covered the low-lying areas; the seas 
begin to swarm with aquatic forms of immense size 
and great strength. Simultaneously with these there 
came in those that could live on the land as well as in 
the water. These were obviously higher in point of 
being and possessed wider adaptations. 
Life was progressing; but it did not halt there. 
Ho sooner had some creatures become at home on the 
