92 Transactions Texas Academy of Science. — 1906. 
embryo producing special organs mainly for protection, respiration 
and excretion, instead of a motile larva forming organs directed to- 
wards locomotion and nutrition. 
Complicated as some of the preceding life cycles are by themselves, 
they become still more intricate by entering into various combina- 
tions. Complex life cycles are well known in the Dicyemids, Cesto- 
des, and digenic Trematodes. But it appears to have overlooked that 
just as complicated series are to be found in the Insects. For in some 
of the latter there is a resistent egg shell or chorion within which the 
embryo developes an amnion and serosa ; that is ekdytic development ; 
after hatching the larva becomes a pupa, and this gives rise to the 
adult by budding: that is metagenesis; then .the succeeding imago 
may generate by unfertilized eggs and a later one by fertilized eggs: 
that is heterogenesis. To analyze complex life cycles into their vari- 
ous component processes should be one of the major aims of future 
embryological study. 
C. The Biological Unit. 
When one considers the various elements that enter into life cycles, 
the question naturally suggests itself : what is the unit here ? Much 
might be written upon this matter, with long historical digressions 
and comparison of the other sciences, but without going to any such 
extent I would plead here very briefly for the right to consider the 
life cycle as the most expressive biological unit. 
One of the earliest views was to consider qualities (as separate from 
structures) as the units composing living beings; this was the idea 
of Aristotle, and this is again what the modern students of hybridi- 
zation are maintaining. With the spread of the Mosaic doctrine of 
Genesis the species came to be considered the unit, with the idea that 
there are so many of them to-day as there were in the beginning, and 
that the species as a whole was established at once. With the founda- 
tion of the theory of transmutation it followed that species are ar- 
bitrary groups, absolutely arbitrary if evolution is to be regarded as 
perfectly continuous, so that the idea of the individual changed first 
to the whole individual, using this word in the signification of person, 
second to some component of the individual. Before the time of the 
evolution theory Cuvier had employed the organ as the structural 
unit. Then after 1839, the cell has been generally granted that 
position, and perhaps to most biologists to-day the cell remains the 
physiological and structural unit. Later still portions of the cell 
came to be considered the units, as the pangenes of Darwin and de 
Vries, the bioplasts of Altman, the physiological units of Spencer, 
