If you don't know, Duane Gish is a creationist, and this article apparently came from the Creation Research Society Quarterly way back in 1989. It's a classic example of the pathetic and illogical arguments of the creationists, which may explain why the Creation Research Society Quarterly, assuming it really exists, is not a peer-reivewed journal.
Gish refers to a research article that suggests the squirrel subspecies haven't diverged enough to have been separated for the millions of years the Grand Canyon has supposedly been in existence. According to him, this proves that the Grand Canyon itself is young.
If the Grand Canyon was formed during the waning stages of the Flood, as receding Flood waters drained from the emerging North American continent, there would have been no squirrels on either rim of the newly formed Grand Canyon. It would be many years after the formation of the Grand Canyon before squirrels and other animals could have arrived. It appears more likely that the tassel-eared squirrel migrated to areas on both sides of the Grand Canyon and that these areas have since become ecologically isolated from one another in relatively recent times. Evolutionists, of course, assume that this isolation occurred several million years ago, whatever the causative factors.
The illogic of this is clear. Gish clearly states that the squirrels had to have come to the Grand Canyon after it was created, but then argues this implies a recent creation for the Grand Canyon. But the arguments that the squirrels came (a) after the canyon was formed, and (b) recently, gives us no indication of how long ago the Canyon was formed, except that it was before the squirrels got there. The squirrel evidence suggest neither a millions year old Canyon nor a thousands year old one. Elementary logic-Gish's weak point-shows that the two events are simply not related to each other.
And when Gish says "Evolutionists...assume that this isolation occurred several million years ago..." he is simply lying. Geologists have evidence that the Grand Canyon is millions of years old. Evolutionary theorists don't assume the age of the canyon is necessarily related to the date of squirrel divergence. If the evidence shows that squirrels diverged only in the last few thousand years, then that's what the "evolutionists" believe. Then they ask themselves, "If the Grand Canyon is millions of years old, and the squirrel divergence between North and South Rims occured only a few thousand years ago, what does that mean? It means the squirrels migrated to the North and South Rims after the Canyon was created." So as it turns out, Gish actually agrees with the "evolutionists" on that point.