You can use the sample.split() function from the caTools package for this purpose:
Start off by loading the 'caTools' package:
library(caTools)
Then, use the sample.split() function which takes in two parameters -> the dataset - 'beaver1' and SplitRatio - 0.65
sample.split(beaver1,SplitRatio = 0.65)->mysplit
Following which, use the subset() function and select all those observations where 'mysplit' tag is True and store them in 'train'
subset(beaver1,mysplit==T)->train
Similarly, select all those observations where the 'mysplilt' tag is Fasle, and store them in 'test'
subset(beaver1,mysplit==F)->test
And that's how you split the data into 'train' and 'test' sets.