198 DR W. B. DRUMMOND ON 
very interesting and striking fact that in our own day, 
amongst the most highly developed fishes, examples of these 
ancient families should be able to maintain a place. When, 
however, we try to go further back, and ask whence these 
primitive fishes came, and whence the great backboned 
family, we find that our knowledge is still very incomplete. 
Hints, indeed, we get in the structure of the lowest verte- 
brates, in the life-history of the Ascidians, in the structure 
of some strange forms, such as Balanoglossus; but, on the 
whole, all we can safely affirm is that there has been no 
break between the vertebrate and the invertebrate series. 
We have already noted in the pelagic fauna examples of 
nearly all the great groups of invertebrate animals. When 
we ask what the geological record has to say as to the origin of 
these, we come upon one of the most interesting facts that 
geological research has made known to us. We find that 
in the oldest fossiliferous rocks known to us, the lowest 
rocks of the primary or palezoic series, the Cambrian, all 
the chief classes of the invertebrata are already represented. 
The corals, the crinoids, the asteroids, the arthropods, the 
cephalopods of the Cambrian series are just as distinct from 
each other as are their modified descendants of our own day. 
This is, as I have said, a remarkable fact, and one reason 
why it is remarkable is the light it throws on the immense 
antiquity of life on the earth, for the existence of these 
varied types of invertebrate forms postulates a period before 
the oldest fossiliferous beds were laid sufficiently long for 
their development. 
Here, then, the geological record fails us, and we have 
to turn to our other book. The embryological history 
of these groups points plainly to a time when star-fish, 
molluscs, arthropods did not exist as such, but were 
represented by free-swimming forms. We have already 
noted among the surface fauna numerous larval forms, 
free-swimming creatures, as transparent as glass, which 
differ so greatly from the adult type that it may be 
impossible on examining them to decide even the class to 
which they belong. The abundance of free-swimming 
larvee among marine animals plays an important part in the 
diffusion of the species, and when we examine these larval 
