Ipsilateral visual projections in non-eutherian species: random variation in the central nervous system?

The published descriptions of ipsilateral visual pathways in non-eutherian species are reviewed. Such pathways exist in members of all vertebrate classes; since they exist in agnathans, it is suggested that the presence of ipsilateral visual projections is the ancestral condition. None of the published attempts to explain the considerable interspecific variation of these pathways can be generalised to all vertebrate species: in particular, this variation is not generally related to the degree of overlap of the visual fields, to a particular mode of life, nor to taxonomic position within a given vertebrate category and cannot consistently be explained by variation at the albino locus. It is suggested that this variation is the result of purely random variation of unidentified elements of the genetic material or of epigenetic mechanisms and hence that ipsilateral visual projections are functionally neutral. This conclusion is supported by some extremely fragmentary behavioral data indicating that the information they provide is redundant.