TRANSACTIONS OF SECTION D. 851 
or tissue must involve modification, at least in functional activity, of other 
organs. He then continues by urging that one organ may replace or be substituted 
for another, the replacing organ being in no way derived morphologically from 
the replaced or preceding organ, but having a genetic relation to it of this kind, 
that it can only arise in an organism so constituted, and is dependent on the prior 
_ existence of the replaced organ, which supplies the necessary stimulus for its 
formation. 
As an example he takes the axial skeleton of vertebrates. The notochord, 
formed by change of function from the wall of the digestive canal, is the sole 
skeleton of the lowest vertebrates, and the earliest developmental phase in all the 
higher forms. The notochord gives rise directly to no other organ, but is gradually 
replaced by other and unlike structures by substitution. The notochord is an 
intermediate organ, and the cartilaginous skeleton which replaces it is only intel- 
ligible through the previous existence of the notochord; while, in its turn, the 
eo skeleton gives way, being replaced, through substitution, by the bony 
skeleton. 
The successive phases in the evolution of weapons might be quoted as an illus- 
tration of Kleinenberg’s theory. The bow and arrow is a better weapon than a 
stick or stone; it is used for the same purpose, and the importance or need for a 
better weapon led to the replacement of the sling by the bow; the bow does not 
arise by further development or increasing perfection of the sling: it is an entirely 
new weapon, towards the formation of which the older and more primitive weapons 
have acted as a stimulus, and which has replaced these latter by substitution, 
while the substitution at a later date of firearms for the bow and arrow is merely 
a further instance of the same principle. 
It is too early yet to realise the full significance of Kleinenberg’s most sug- 
gestive theory; but if it be really true that each historic stage in the evolution of 
an organ is necessary as a stimulus to the development of the next succeeding 
stage, then it becomes clear why animals are constrained to recapitulate. MKleinen- 
berg suggests further that the extraordinary persistence in embryonic life of organs 
which are rudimentary and functionless in the adult may also be explained by his 
_ theory, the presence of such organs in the embryo being indispensable as a stimulus 
to the development of the permanent structures of the adult. 
| It would be easy to point out difficulties in the way of the theory. The 
- omission of historic stages in the actual ontogenetic development, of which almost all 
‘That recapitulation does actually occur, that the several stages in the develop- 
“ment of an animal are inseparably linked with and determined by its ancestral 
history, must be accepted. ‘To take any other view is to admit that the structure 
of animals and the history of their development form a mere snare to entrap our 
judgment.’ 
Embryology, however, is not to be regarded as a master-key that is to open 
the gates of knowledge and remove all obstacles from our path without further 
trouble on our part; it is rather to be viewed and treated as a delicate and com- 
licated instrument, the proper handling of which requires the utmost nicety of 
balance and adjustment, and which, unless employed with the greatest skill and 
judgment, may yield false instead of true results. 
Embryology is indeed a most powerful and efficient aid, but it will not, and 
€annot, provide us with an immediate or complete answer to the great riddle of 
