156 PAST AND PRESENT 
ward and study the fossils of older and older rocks, 
the multitudinous assembly of plants and animals 
which fill the world to-day are replaced by other and 
more primitive forms, many groups approaching each 
other and merging in common ancestral types. But 
still, the very oldest fossil-bearing strata contain the 
remains of organisms already far up the ladder of 
evolution. The Lamp Shells (Brachiopods), Ptero- 
pods, Trilobites, and Worms of the ancient Cam- 
brian rocks have clearly a long ancestral history. 
Plants are not so abundantly preserved in the rocks 
as the skeletons and shells of animals, on account of 
their softer nature; but in the oldest known plants it 
is again clear that we are dealing with forms by no 
means primordial. It is the more interesting, then, 
to note that many very lowly forms of life have come 
down to us from times immensely remote, and are 
still present on the earth in abundance, swarming in 
every sea and in every pond, or nestling in damp 
crevices of the land; while higher types of immense 
antiquity still mingle with the crowd of recent Seed 
Plants, some of them forming noble forest trees. Of 
especial interest, taking into account the wide dis- 
tinction which exists between the higher animals on 
the one hand and the higher plants on the other, is it 
to find that there are still in existence organisms 
which are so much on the border-line between these 
two great groups of living things that they can be 
referred to one or other only with hesitation, 
clearly indicating that animal and vegetable life sprang 
from a common source. Take the group known 
as Mycetozoa or Myxomycetes. These names alone 
show the divergent views which men of science 
